Forms doesn't have relationship, tables have.
As you describe it, I'd say tblOrder on the 1 side, tblFaxes on the M side -> primary key from tblOrder becomes foreign key in tblFaxes.
A separate form is not linked in any way to another form, so what you probably need, is some means of assigning the primary key value from the current record in your form bound to the order table per each new record in your pop up form.
one could use the openargs to pass it, or for instance in the on current event of the form bound to the faxes table, pick up the primary key if it's a new record. Might look like this:
[tt]if me.newrecord then
me!txtFK.value = forms!frmNameOfOrderForm!txtPK.value
end if[/tt]
where txtFK is the control on the form bound to the faxes tables foreign key, and txtPK is the control bound to the primary key field on the order form.
Roy-Vidar