Look at this for the sales order screen.   Same basic procedure for the return screen.  Which should be SRH.

(+) Default quantity - General Discussion - Sage X3 - Community Hub

is it the right place?

return value i want to default to blank. I was thinking that there will be parament for that. 

The value in the QTY column is set in a couple of places, but will be set automatically when a line is picked from either:

1. A Delivery

2. A Supplier Return

There is no parameter driving this behavior, the best would be to make use of entry point ALIPICSRH in SUBSRHC to reset the value of [M]QTY to 0. 

Could you please provide a screenshot demonstrating how to locate 'ALIPICSRH' and 'SUBSRHC'?

I think that will work.   Don't forget about validation, might need to validate the window too and transaction.