Add a textbox value to a value in a table

I know this is a basic question, I have a form where there is just a combobox and a textbox, the user uses the combobox to select the record they want to change and then they enter a value in the textbox which needs to be added to a field in that record.



this is the vba I tried to write with my very limited knowledge:

Code:

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StockNumber.Value = (StockNumber.Value) + (DeliveryValue.Value)

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I get a runtime error "object required" and I have no idea what to do.

The textbox is called DeliveryValue and the record I want to add it to is called StockNumber in a table called Products.

Add a textbox value to a value in a table