I can't seem to set a layout rule to achieve this, because this functionality doesn't seem to extend to lookup fields.

Essentially, there's one field where a user is supposed to select a value from a lookup field that offers 50-60 options. In case they can't find the correct item to link to, there is also a free text field where they can enter it. 

I'm trying to think of a way to make it so that the free text field only becomes mandatory in case the lookup field was not filled in.