Classic Projects: Add Client PO Number in "Create Classic Project" endpoint

Hello:

I can see that when adding a task to a project, there is a field for the Task PO Number: 

"clientTaskPONumber"

That's for the endPoint: /projects/{projectId}/tasks

But isn't there a way to insert the Client's PO Number in the project scope?
[POST] projects/ doesn't have a filed for it.

Thanks for you help.