Can you put a private plate on with a new keeper slip online?

No. The DVLA's online private plate assignment service requires information from the vehicle's V5C log book and will not accept the reference number shown on a green new keeper slip.

This is because the online service updates the vehicle's registration records and issues a replacement V5C as part of the assignment process. If you have only recently purchased the vehicle and are still waiting for the V5C to arrive, you will need to wait until it has been issued in your name before assigning the registration online.

If you do not want to wait for the replacement V5C, it may be possible to apply by post using the green new keeper slip together with a completed V62 application.