Hi, thank you for asking!
When you're on your account and find your account shairng link, the characters after /user will be the user id. Hope this helps!
1.Open the desktop spotify application
2.Click on your name on the upper right corner of the screen
3. A screeen with your name and bunch of playlist will show up. Click on the circle with the three dots in them. (...)
4.Click on "copy profile link"
5.Open up a browser and paste it into the search bar
6.the number that shows at the end of the url is your Username
You can also go into your account in a web browser and click "Set device password" on the left-side menu. It will display your device username (mine is a string of 10 numbers). I found this after far too long a search...I'm looking at you, Spotify. -_-