You don't need a field at all then. Ensure substitutions are enabled in your auto action under the Parameters tab then add the following as the HTML output for your Code auto action.
Code:
[cb:usertab tab="46" user="[var2_user_id]" /]
var2 is the group object so you can substitute any part of the group using that. In this case it's setting the substitution user to the group owner user id.
krileon
