SAML Authentication with Okta Identity Provider

I have setup everything for saml authentication with Okta. By default it gives NameID as application username but i want to use employeeNumber.

So i have also done with the mapping between okta user profile -> app user profile. Like -

user.employeeNumber(okta user profile) -> userName(app user Name)

But still i am getting app username in SAML assertion but not the employeeNumber.

Can anyone tell me how can this can be achieved or if i am missing something?