linking subform to mainform field

I have a main form that has a field called LastName. I also have a subform
that also has a field called LastName. I placed that field in the subform to
be able to correlate the subform to the mainform BUT what I want is that each
time I open the form to a specific LastName in the main form that the subform
field called LastName will auto populate with the same value in the mainform
field called LastName. How do I do this? I have done this a long time ago
but can not remember how I did this. I recall I did this completely using
the Build option in properties of the field and did not use VBA ( I am not
versed in VBA and would prefer to not do this in VBA). Can anyone assist?