AM20.3 add-ins upload to add serial number on each line

We need to be able to add serial numbers to specific line items on an asset in AM20.3.  Does anyone know how to do this with add-ins since the field name for serial is always row0?  We have assets that can have up to 100 line items and each one needs its own serial number.  I haven't tried it yet but the user did and was unsuccessful so I figured I would start here
 
 Thanks!