While you might technically be doing a "POST" request in fiddler (Which I've not used, but I do know a thing or two about HTTP), it looks to me very much like you are not actually posting your query, rather you are using a "PUT" (presumably combined with an empty post).
I believe that if you were to use curl http://localhost:49301/api/donut/run?Task=bake it would work fine.
[ A PUT request puts the parameters as part of the URL, a POST request instead puts them in the request header.
Its probable that your application is programmed to accept PUTS but not POSTS.
