Profile Enrollment

Hi, I am trying to do the following, the user registers in Okta, as username we have set his personal identification number profile enrollment tells me that this attribute has to be read-write mandatory to be able to write it.

The question is that I don’t want the users to be able to change this attribute later, I want them to add it only when registering and then I want it to be read-only.

I made the following tests, correct me if I am wrong, I put the attribute username as read-write. I register with a username, firstname, lastname and email, all correct, in the end user panel, I go to settings, edit profile, I see that username is still as read-only even if they are read-write, is this a normal behavior, if so and Okta does not let serial change the username to the users it would work for me. If not, I don’t know why it doesn’t let me edit and if it has any other setting behind.

Thank you very much, best regards.