Behaviour- set current user in a User-field but display only his full name

[Dan27]

Hello,

I am using behaviour to set the current user in a field (User picker type) when issue moves to Done (on Done screen).

I want to display only the full name of the current user, But wen I did it in my script, Jira cam't find this user.

This is my script:

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.Issue
import com.atlassian.jira.issue.fields.CustomField
import java.sql.Timestamp

def customFieldManager = ComponentAccessor.getCustomFieldManager()

// Getting custom field object
def CustomFieldName = getFieldById("customfield_XXXXX")

//Get current user
def currentUser = ComponentAccessor.getJiraAuthenticationContext().getLoggedInUser();
def fullName = currentUser.displayName

CustomFieldName .setFormValue(fullName.toString())

How can I do this display without distroying the functionallity?

Attached print screen of the behaviour:

Thank you!

Daniel