I would simply write it this way:
f(; a=1, b=2, unused_args...) = (a, b)
But of course this function is misleading as-is, since it does not
complain if you pass unused arguments. A more common/useful situation
would be something like:
g(; a=1, b=2, h_args...) = (a, b, h(; h_args...))
h(; c=3) = 2 * c
Regards