How can I pass a value to restrict a picklist from a subform when adding a new record to the lookup form that populates the picklist?

How can I pass a value to restrict a picklist from a subform when adding a new record to the lookup form that populates the picklist?

For instance if I had a main form that contained the field "Country" 

1. and a subform in which I needed to add a new entry into one of the subform's field's picklists

2. and the lookup form that populated the subforms picklist had both a "Country" and "City" field,

3. how do I filter the lookup's "City" field by the "Country" selected in the main form? 

Or in other words how do I pass a the "Country" variable value 3 levels deep to the lookup field. I can easily do it if it's a lookup for a field in the main form.