How to default CPA on all req lines

Summary

How to default CPA on all req lines

Content

1 We will only be having one CPA  oer req/Po

2. We do not want to auto-source using the CPA flag because we have multiple CPA per supplier site

3. We do not want to auto create PO.

All we want to be able to do is enter the CPA on the first req line and for it to automatically defaut to the other req lines. I can't find a way to do this. Also, the document builder source agreemnt only allows BPA selection not CPA - otherwise this would be ideal.