Setting record source of popup form (1 Viewer)

[TiggR]

I am trying to set the record source of a subform of a popup form which is a part of a collection of the main form. I am having difficulty referencing this as an object.
I have used the tag property of the main form to store the filter which is pulled in by a SQL statement in the subfrm, but I would like to be able to set the recordsource directly.
Please can anyone help?!

 

[TiggR]

My learned colleague has shown me to refer to the subfrm as part of the controls for the frm in the collection.