Here's a module I programmed for controlling volume and basic Spotify playback functions with Flic Twist, in case someone else has any use for it. To use it, you'll have to register for a Spotify developer account and create a new app from their dashboard, in order to get access tokens etc.

@flichub @Emil: Is it possible to somehow get this added to the existing public Spotify "provider"? Seems a bit tedious/difficult for users without any programming experience to go down this particular rabbit hole.

// main.js
const flicapp = require('flicapp');
const http = require('http');
const datastore = require('datastore');

datastore.get('clientID', (err, key) => {
	const clientID = key     // store this first: (datastore.put('clientID', 'YOUR CLIENT ID')

	datastore.get('clientSecret', (err, key) => {
		const clientSecret = key  // store this first: (datastore.put('clientSecret', 'YOUR CLIENT SECRET')

		function percent(volume, decimalPlaces = 0) {
			const percentage = (volume * 100).toFixed(decimalPlaces);
			return `${percentage}`;
		}

		function spotifyStatus() {
			datastore.get('accessToken', (err, key) => {
				const accessToken = key
				if (key !== null) {
					const options = {
						url: 'https://api.spotify.com/v1/me/player',
						method: 'GET',
						headers: {
							'Content-Type': 'application/json',
							'Authorization': 'Bearer ' + accessToken
						}
					};
					const req = http.makeRequest(options, (error, result) => {
						if (result.statusCode == 401) {
							refreshToken();
						}
						if (result.statusCode == 204) {
							var status = "No active sessions."
							console.log(status);
						}
						if (result.statusCode == 200) {
							var jresp = JSON.parse(result.content);
							var status = jresp["is_playing"];
							if (status === true) {
								spotifyPause();
							}
							if (status === false) {
								spotifyPlay();
							}
						}
					})
				}
			})
		};

		function spotifyPlay() {
			datastore.get('accessToken', (err, key) => {
				const accessToken = key
				if (key !== null) {
					const options = {
						url: 'https://api.spotify.com/v1/me/player/play',
						method: 'PUT',
						headers: {
							'Content-Type': 'application/json',
							'Authorization': 'Bearer ' + accessToken
						}
					};
					const req = http.makeRequest(options, (error, result) => {
						if (result.statusCode == 401) {
							refreshToken();
						}
					})
				}
			})
		};

		function spotifyPause() {
			datastore.get('accessToken', (err, key) => {
				const accessToken = key
				if (key !== null) {
					const options = {
						url: 'https://api.spotify.com/v1/me/player/pause',
						method: 'PUT',
						headers: {
							'Content-Type': 'application/json',
							'Authorization': 'Bearer ' + accessToken
						}
					};
					const req = http.makeRequest(options, (error, result) => {
						if (result.statusCode == 401) {
							refreshToken();
						}
					})
				}
			})
		};

		function spotifyNext() {
			datastore.get('accessToken', (err, key) => {
				const accessToken = key
				if (key !== null) {
					const options = {
						url: 'https://api.spotify.com/v1/me/player/next',
						method: 'POST',
						headers: {
							'Content-Type': 'application/json',
							'Authorization': 'Bearer ' + accessToken
						}
					};
					const req = http.makeRequest(options, (error, result) => {
						if (result.statusCode == 401) {
							refreshToken();
						}
					})
				}
			})
		};

		function spotifyVolume(volume) {
			datastore.get('accessToken', (err, key) => {
				const accessToken = key
				if (key !== null) {
					const options = {
						url: 'https://api.spotify.com/v1/me/player/volume?volume_percent=' + volume,
						method: 'PUT',
						headers: {
							'Content-Type': 'application/json',
							'Authorization': 'Bearer ' + accessToken
						}
					};
					const req = http.makeRequest(options, (error, result) => {
						if (result.statusCode == 401) {
							refreshToken();
						}
						if (result.statusCode == 404) {
							console.log("No active sessions.")
						}
					})
				}
			})
		};


		function refreshToken() {
			datastore.get('refreshToken', (err, key) => {
				const refreshToken = key
				if (key !== null) {
					const options = {
						url: 'https://accounts.spotify.com/api/token',
						method: 'POST',
						headers: {
							'Content-Type': 'application/x-www-form-urlencoded',
						},
						content: "refresh_token=" + refreshToken + "&scope=user-read-playback-state%2C+user-modify-playback-state%2C+user-follow-read%2C+user-library-read%2C+streaming%2C+user-read-playback-position%2C+user-top-read%2C+user-read-currently-playing%2C+&client_id=" + clientID + "&client_secret=" + clientSecret + "&grant_type=refresh_token"
					};
					const req = http.makeRequest(options, (error, result) => {
						if (!error && result.statusCode === 200) {
							var jresp = JSON.parse(result.content);
							datastore.put('accessToken', jresp["access_token"])
							spotifyStatus();
						}
					})
				}
			})
		};

		flicapp.on("actionMessage", function(message) {
			if (message == 'playtoggle') {
				spotifyStatus();
			}
		});

		flicapp.on("actionMessage", function(message) {
			if (message == 'next') {
				spotifyNext();
			}
		});

		flicapp.on("virtualDeviceUpdate", function(metaData, values) {
			if (values.volume && metaData.virtualDeviceId == "spotify") {
				var volume = percent(values.volume)
				spotifyVolume(volume);
				flicapp.virtualDeviceUpdateState("Speaker", "spotify", {
					volume: values.volume
				});
			}
		});

	}); // clientID var
}); // clientSecret var

As you probably can see, I'm not exactly a Javascript wizard, but it works Enjoy.

Andreas