That GUI field is for the trunk registration, and you are looking to set it for the trunk endpoint. AFAIK, the only way to do this is by looking in the file pjsip.endpoint.conf
to determine the trunk name and then editing the file pjsip.endpoint_custom_post.conf
with:
[trunk_name](+type=endpoint) ; sub actual trunk name between []
contact_user=MrGrinch