Say that you have global fields named Globals::Field1 and Globals::Field2 in your popover.
A "Save" button in the popover might perform this script:
Set Field [LayoutTable::LocalField1 ; Globals::Field1 ]
Set Field [LayoutTable::LocalField2 ; Globals::Field 2]
There might be more needed than that, such as capturing the current record's primary key in a variable, then changing layouts to a layout based on a related table where you then do the new record and set field steps, but then add a set field step that set's the new records foreign key field to the variable's value in order to link the new child record back to the parent record on the original layout.