How can I address a corner missing the window framing baseplate without replacing the windows this year – suggestions or fill with foam/plaster?

Hey everyone! So, I’m knee-deep in preparing for the next round of skim coating, and I noticed something interesting. It turns out that the previous owners of my house widened a window opening back in the 90s, and left the bottom window framing looking a bit questionable.

I’m not planning on replacing the windows just yet, but I do want to make sure that this corner missing the window framing baseplate is properly supported. Any suggestions on how to go about this? Should I fill it with foam or plaster? Let me know what you think!