From the Twitch app:

To copy the link or URL, go to the profile you want to assign the Twitch follower service to.
– click on the 3 dots in the upper right corner of the profile.
– select: "share via.."
– copy it to your smartphone notes
– the link will look something like this:

https://www.twitch.tv/profilename

From the Twitch website:
If you want to copy the link or URL from the Twitch website, simply
– go to the profile you want to assign our services to
– copy the link in your browser's address bar

The following formats are also valid:

– username

– https://comprasocial.me/how-to-copy-twitch-link/