Version 4.4.6 (and V4.4.4 before)
The selection of extra related products is impossible.
I can add the custom field, but when it comes to selecting an 'extra related product' the search feature doesn't work, I type in a product description or part thereof and there are no options to add, none are presented, Ive tried by product name or SKU and nothing is presented.
I have the latest version, and everything works fine for me. It's very likely that your problem is related to a full update of VirtueMart along with the VmAdmin template. I never do this because updating this template usually causes more issues than benefits. Install an older template (i have vmadmin 4.2.18) and simply fix line 54 in the /administrator/templates/vmadmin/html/com_virtuemart/custom/default.php file by changing:
$customs = $this->customs->items;to
$customs = $this->customs;