Move the first record on to a separate worksheet.
Go to the Main Layout, select File -> Import records, locate the spreadsheet, and import just the single record.
Swith to a layout based on the portal's table, select File -> Import records, locate the spreadsheet, and import list of records.
(There are other ways, but that's the most straightforward).
Don't forget that you will doubtless want the portal table's records to all include the Key Field that you use to link them back to records in the main table.
You can script that, of course, if you're going to do it repeatedly.
You can also specify a range for the first record and a different range for the rest of the spreadsheet, then do your two imports by specifying different ranges for each.
However, given that this is just one field of just one record, you might also just import it all in one go and then go to the first record of the found set produced by the import and copy that first field's data to the place it needs to go before deleting this single first record.