Custom Username for WebApplication

I’ve some troubles defining the username format for registered application.

I’ve created my application in Okta dashboard as:

Platform -> WEB
Sign-on Method -> Secure Web Application

After that I’ve specified Login URL e Login Redirect URL.

About the “Sign-on Methods” section, here is my configuration:

For Username I would like to use this custom format:
first letter of firstName merged with full lastName, all in capital letters.

Something like this should work?
{f:substring(f:toUpperCase(user.firstName), 0, 1)}{f:toUpperCase(user.lastName)}

Why when I try to login throught the link generated in the “General” section, the only username accepted is the user’s email?