How to pass a query param or flag with no value?

[jporub]

My target url looks like this:
/api/v1/Dinosaur?XYZ&foo=1234
As you can see XYZ is a param with no value, the endpoint doesn't take XYZ=1 it only accepts XYZ, don't ask me why, it's just a legacy thing.
I even tried the following but it strips out null or empty params and discards it:
[Post(/api/v1/Dinosaur?XYZ)
Task DoStuff(int foo)

I even tried the above but it strips out null or empty params and discards it.

Any ideas on how I can get my outgoing URL to look like my target up top?