A PO number only gets generated once a PO has been submitted.

You will still need to raise a PO but you can create a placeholder PO with a zero amount and amend it when you have all the information.

You can also create a supplier called ‘Placeholder’ to assign all of these POs to while waiting for all the information. You can then filter out these placeholder POs by filtering by Supplier Name.  

Note, the PO will go to the first user in the approval chain to approve, a way around this would be to put whoever is raising the PO to generate a number as the level 1 approver to avoid it progressing in the approval chain.