How to populate text field from entity reference

The feature is not available. You would have to write custom Ajax code to accomplish it.

Hi @jrockowitz thank for your reply.
i'm not a programmer, i try to add this in Webform > Settings > CSS/JS:

jQuery(document).ready(function() {
  // Get the autocomplete element and the textfield element.
  var autocompleteElement = document.getElementById('edit-username-autocomplete');
  var firstNameElement = document.getElementById('edit-field_first_name');

  // When the user selects a username in the autocomplete element,
  // update the first name element with the user's first name.
  autocompleteElement.addEventListener('change', function() {
    var username = autocompleteElement.value;
    var user = Drupal.user.load(username);
    if (user) {
      firstNameElement.value = user.field_first_name;
    }
  });
});

But i got this error in console:
Uncaught TypeError: Cannot read properties of undefined (reading 'load')
at HTMLInputElement.

I hope you can help ;-)

and thank you again.

For help with programming, look into the resources I provided in comment #2.