It cannot be remapped in-game, however you can edit the InputBindings.json to add it manually. The file can be found in:
%localappdata%low\Intercept Games\Kerbal Space Program 2\Global
The file should end with }]}". After the first of the two curly brackets, add a comma, and paste in the following:
{\"action\":\"ToggleAeroGUI\",\"id\":\"1411ef0d-0b4c-4d4a-8d07-a23d96dea6d2\",\"path\":\"<Keyboard>/f11\",\"interactions\":\"\",\"processors\":\"\"}
In this example, I've bound it to F11, though you can choose any other key that the game supports. If you're unsure what to type for special keys, you can always bind something to that key in-game, then check this file after applying.
It seems (some?) custom bindings aren't loaded on game launch, so every time you launch the game you'll need to go to the Input settings and click the Apply button.