I don't see why you need an "Add to stock" button at all, Simply getting the records in the table of the portal at the bottom should have done that already.
But then I don't recommend that you use your current structure for managing inventory either. Your current design approach requires that you define new fields(QuantLoc1, QuantLoc2...) and update your system to use them in order to add a new location should such a change become necessary.
I suggest using what I call an Inventory Ledger to log items in and out of inventory. A field in such a table can record the location to which you are adding or removing items. Your BookIn table is very close to this ledger type approach already.
See this thread for more about this method: Managing Inventory using a Transactions Ledger
Yeah, I went into it without really knowing how I was going to achieve the end result; never a good idea (PPPPP).
Thank you for the link to your other post.