I dont know if I understand correctly, but I will try to help.
So you have a column in the “Shopify Orders” that relate to the “PO” table, right? Is it a single relationship or multiple? You can add new values from the relation?
What do you mean when you say “When I click to add a new PO”? Is it just the default new row button (on the bottom, on the top, or right clicking), or is it when selecting the PO rows in the Shopify table and you add a new data, or is it a custom button that adds a row?
To automate the PO generation, you can add a formula to the column (like “PO” + thisRow.RowId()). This will block the edition to this column. You can use also the “Value for new rows” in the column options with the same formula. In this case, as soon as the row is created, it will fill the column value with “PO#”, but you can edit it at any time.
I think (i not tested it) that if your case is that yoiu want to add a new PO from column in the Shopify, it will not work as expected. I dont know how coda will work in this case.
So, if you could explain a little bit more, we can manage the best approach.