You appear to be familiar with triggers but just in case here are a few articles you might find useful:
Triggers -- SQL Server[
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Overview of SQL Server database Triggers[
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Be Very Careful When You Write SQL Trigger[
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You have mentioned using cursors but not why you might think you would need one - however all you need to do is join to the
Inserted
table within the trigger ... e.g. something like this perhaps
UPDATE A
SET A.Quantity = A.Quantity + C.Quantity
FROM OutgoingStockProducts A
INNER JOIN Inserted B ON A.FormID = B.FormID
INNER JOIN Stock C ON A.StockCode = C.StockCode
If this is way off the mark then comment below and I will try to help further