The User-Defined tab will allow you to create the string which the solver input deck will use to invoke the user subroutine.  This tab also allows you to optionally select a specific .dll file that contains the compiled user subroutine, as well as an optional user subroutine name.  Place a check mark in the User-defined properties check box on the Connectivity tab to activate the User-Defined tab.

Spring Dampers panel - User-Defined tab