Access custom field in user profile in rules

Hi!

I have a content type with a field of type "Entity Reference" and a Flag field.
The Entity reference has a list widget so I can add usernames to it.

In my user profile I added a custom field named "realname" to have the real name of each user.

What I want in rules is to add the realname to the list and not the username when I click the flag of my content.
At the moment it works but the value added to the list is the username and not my custom field "realname".

I'm unable to reference this custom field in my content type. What am I doing wrong?

Thanks,
zephirus