How to populate the preferred_username claim?

[Vandaahl]

I want to integrate self hosted Logto as an IDP for Affine, which relies on the preferred_username claim when logging in. If Affine does not receive preferred_username, login fails. I read in the Logto documentation that preferred_username will be provided in the user profile IF it is not empty. So me being new to Logto, my question is how do I populate preferred_username?

[Vandaahl]

So if I open the Logto admin and go to User Management and edit a user, under User Data I can add this to the Profile field and Affine login starts working:

{
    "preferredUsername": ""
}

But still, adding that manually to each new user is not optimal. I need to automate it somehow.