This seems related to a topic and further posts you created last year at Adding- autofill discount option in customer/supplier especially post 18:
It was made clear in post 8 by @Lubos that Manager does not have this capacity until:
Also when trying to write and extension even though as you acknowledge an obsolete feature and thus not supported that to have any success in getting support that you should show us your attempt in code and where relevant screenshots if to get any help with coding.
I flagged your post because it puts 2 issues in one topic and therefore is much better to have 2 separate topics to prevent total confusion as to which of the 2 topics is under discussion, for example I am now replying to your second issue.