I think it will be RedBull itself, as Marshall was working with Red Bull on the 2026 engine project, among other things.
I know it wasn't exactly with the development of the engines, but a combination of how they should fit into the car, or something like that.
In his new role at McLaren, Marshall appears to be ready to take responsibility for securing this partnership, and during an event that took place for the Spanish press this was even asked... whether Marshall could be linked to the possible deal regarding the engine with Red Bull from 2026.
The answer was not conclusive, he said that a partnership with Red Bull is not imminent, but he did not deny it, which could be a good sign and I believe that this partnership will indeed happen.