Adding a sill to a Marvin Integrity window
I happen to have a shop built with Marvin Integrity windows. As far as I know, the customary/expected way to provide an interior sill/trim is that it either butts up to or slips behind the wood frame provided with the window. Yes, there's a visible seam - it's not a big deal.

Your photographically-challegened-ness makes it difficult to determine if you already have the most minimal form of this installed, or not. Mine are currently sitting with no interior trim, waiting for my "tuit" to get "round."