Hi,
I'm sorry but I did not understand. could you explain me better?

Hi,
sorry for late reply, it was night for us.

This is not possible with a simple way, and I can't guide you to the better solution because I does not understand well your purpose.

Also keep in mind that if a field is set to readonly then it will be not stored also when it is filled with a default value.

I want this because I don't want to expose the user names in the site registration form.

To make this there is a very simple solution:
1) Fill username: you can make this from url like https://www.your-domain.com/registration-url.html?username=test
2) Hide username: you can add CSS like this

#member-registration .username-group{display:none !important}

There is also a simple way to hide username, you can set "Login Type" to "Only Email" in Easy Profile Options. This way also hide username in Edit Profile page

Hello Thank you for your reply.
I want to use this field as some kind of referral, but I don't want to use the user name but the ID, so I prefer something like ?username=500

I create a dropdown list with options for data base but with this way I expose all users in registration form, by expose I don't mean to the user try to register but if someone take a look on the source code will see all the users.

I think just fill the userid in some text field in registration form and then some other field in profile took this value and show the username, but as you say this is not possible.

Anyway I think different way to do that I want.

Thanks.

I think just fill the userid in some text field in registration form and then some other field in profile took this value and show the username, but as you say this is not possible.

I mean the second field username will took data from DB user table and will filter by userid field, but then the username with ID same with userid value is the first option but not selected. That I ask how I can select by default the first option.

mmh...you can do this with a javascript like

jQuery(document).ready(function($){

	var alias = 'here_the_alias_of_select';

	jQuery('#member-registration #jform_'+alias+' option:nth-child(2)').prop('selected','selected');

	jQuery('#member-registration #jform_'+alias).chosen('destroy');

});