Hi Ahmed,
Thanks for your quick response, it works fine on Postman.
It would be a Server.Transfer
or Response.Redirect() at the Client End(Payment Gateway), to which if its a traditional web page, lands absolutely fine, Reactive one throwing a 405 error.
Kindly correct me If I understood it wrong, as I have already tried implementing and struck for the Alternative.
Now that I understood I might have mentioned the title of my question better.