Hmm... looking at this picture more, I'm not really sure what car it's supposed to be. The above mentioned suggestions aren't bad, but I have my doubts if the car in question actually is any of them. I'm puzzled in particular by these three elements in combination, highlighted in red, green and yellow:
View attachment 1204838
First, the red. It's like this strange bar-like thing stretching across the middle of the side. Don't think any McLaren has got anything like that in their design language.
Secondly, the green. It looks like the "upper" part of the side under the window is somewhat "square" yet smooth, and the rear doesn't seem to have a tail light stretching itself to the rear wheelarch. The Daytona SP3 fits the description of the former but not the later.
Thirdly, the yellow. The rear quarter window... I'm not sure if it stretches further backward or if it loops itself back in a nice bezier curve fashion, though naturally the poor resolution of the image doesn't help. In any case, 765LT does the former, GT Black Series the latter.
TLDR; I don't know what car it could be. It could be a car already existing in the game, it could be a car that isn't in Forza yet (even if this one is potentially implausable). Still fun pondering the question/mystery though.