Can I complete the PO form without a PO number?

Yes - but only if your district needs an estimate/invoice prior to issuing a PO number.  Remember: you will not receive access until the PO is paid, so if possible, the fastest way to get the ball rolling is that have a PO number at the start.

Please do NOT fill out a PO or group enrollment form just to "see if your district will pay for it".  That only serves to confuse accounting offices in your district.

The better thing to do is to forward the link to the course, conference, membership, certification, or curriculum that you'd like to your administrator and ask them if they'll pay for this with a PO.  If they agree, ask your school secretary to place the order.  It's less stressful for you and it ensures the process goes quickly.