IOS Keychain Touch Id


I want to make authentication for my app by using the touch id.
So to do this I have to link the fingerprint with a valid username and password.
In my app I want to have a login page with username and password and additonally an option for make authentication with touch id. For the first time I have to login with username and password and want to link it with the touch id by using the function from Keychain Touch Id plugin.

set(key, value, useTouchID)

The key is the username and the value is the password.
But I don’t understand how the touch id will be linked with the username and password.
Can anyone help?