When you initially sign up to Linktree, you can choose your own custom URL on the sign up page by entering a custom username. If you choose the 'Sign Up With Instagram' option, your Linktree username will automatically be chosen as your Instagram handle.
Unfortunately once you have signed up to an account, you are unable to customise your URL beyond this. We are working on a way to integrate this custom URL / domain feature into the Linktree platform.
There isn't an ETA on this feature being available. We are working hard to implement it as fast as possible.