Hello matte,
Here are the steps: (Assuming you already added your list as a Datasource)
0) Create your input form and add all your fields there.
1) In the serial number CARD, which you want to have the data read from a camera, insert a barcode scanner object. (Insert/Media/Barcode scanner)

2) on the OnScan property of that Barcode Scanner, set a temporary variable to receive the data you are reading. Set(tmpScan,Self.Value)
3) On the serial number Card, On the Default property, set the following formula:

Basically it checks for the form mode. If is New then it gets the value from the tmpScan variable, otherwise it gets from the form itself)
4) Finish to add your data and then submit the form.
Let me know if that is what you were looking for.
Cheers,
Leo Couto