What should be my uscis status for filing I-485 when under h1b extension (pending)

You are in a stay authorized by the Secretary because you filed your H-1B extension before your H-1B expired. So, technically, you will not have a status until the H-1B is approved. In this situation, I have put H-1B and on expiration I put authorized stay and submit a copy of the receipt notice for the extension application. I have also had some clients that upgraded to premium processing so that they can have an expiration date to fill in.

Extension pending.

Status is authorized but not lawful beyond I-94 period. best Wishes!