If ping is successful and if backend request is reaching successfully, then endpoint url's certificate is already there in SMP keystore (as its HTTPS based)
I also try to POST direct on back end . But it also give same 403 forbidden .
So here is the problem, first you have to solve this and then you can try via SMP.
You have to get involved with Gateway and backend team on the POST request you are trying. Hope they will be able to help you.
Regards,
JK