Danke again Bamboogypsie,
On Screenshot 2, this field is not editable.
I see that your screenshots are for Ax 2012, I should have pointed out that I am working on Ax 2009. If that makes any difference. (and I think it does, as I far as I can tell, I don't really deal with "product receipt", as there are only packing slips)
Apart from the mentioned above, the process is the same and I get to screenshot 7 without any trouble. My problem is that back on screenshot 2 the field remains empty.