set first in list as initial value (lookup)

set first in list as initial value (lookup)

Hi folks, 

Is there any way that I can set the first value in the list of a lookup field as the initial value? 

The lookup field has a filter set on it, so the initial value will often be different. I just want it to auto-input whatever the first value in the list is. 

I would appreciate any help!

Thank you, 

Lindsay