restrict text entry if text doesn't match field

[AccessNewb1]

As the name implies total newb with Access, I have a form with a text box entry that writes a SerialNumber to a Transaction table. I want to restrict the end user from writing data to the transaction table if the SerialNumber entered isn't an exact match with the SerialNumber in the Products table. Please help as I only started to use Access 2 weeks ago

[Minty]

Use a combo box that lists only valid serial numbers as the text entry point.
Use the wizard to help you set it up.

[AccessNewb1]

I guess I should have noted that the text entry is read from a barcode using a handheld scanner. I am trying to avoid manual data entry by using barcodes.

[kd2017]

Set up referential integrity between the two tables and access won't allow updates to your transaction table if the corresponding serial number doesn't exist in the product table. Google search 'ms access referential integrity'