Creating a Link between form and subform.

Creating a Link between form and subform.

I am trying to link a form and subform. I followed the instructions and it shows you who to use lookup in the subform. Using look up it asks which field in the form you would like to use to link the two. This is a problem, there are no unique identifier fields in the form. I tried using ID which i understand is a long unique field for each record but all I get is a drop down that asks me to select an ID out of hundred of IDs even if search and autocomplete is ticked. 

Can someone show me practically how to link forms and subforms given that there is still no auto number generate function.. 

Thanks