Hello,
We recommend using the following expression: %firstname:upper,1%%lastname%. If this expression is used, the username will consist of the 1st letter of the first name (capitalized) plus the last name. If the last name starts with a capital letter, it will be capitalized.
Otherwise, if you need to force the first letter of the last name to be capitalized in the username, no matter which letter the last name starts with, you'll need to use a script and a Business Rule. If you want, we can help you with the script.