Why always getting bad request fcm? - firebase-cloud-messaging

Question

I am trying to send a notification to the android client from FCM using rest API. My code is: ```
post_offline_message(From, To, Body) ->
K = "FCM key",
U = "https://fcm.googleapis.com/fcm/send",
P = #{
payload => #{
to => <<"rid">>,
priority => <<"high">>,
data => #{
<<"title">> => <<"Some Title">>,
<<"message">> => <<"Hi">>
}
}
},
EP = jiffy:encode(P),
?INFO_MSG("EP data: ~p", [EP]),
httpc:set_options([{keep_alive_timeout, 0}]),
{_, Resp} = httpc:request(post, {U, [{"Authorization", "key=" ++ K}], "application/json", EP}, [], []),
?INFO_MSG("FCM response: ~p", [Resp]).
My EP data (payload) is this:
**<<"{\"payload\":{\"to\":\"dOqZOggYQZG6xKVY9P4_Xi:APA91bG0kuM-o_lSf3fUaWcyiW0fVj8-L49QgzU6rWfxi3o5lMaKapkOjvvLUxm-e78XS49TVl5jjgQt6DrRKTaDK2xzg-ffm1Qe4Xx-61_Hrmr6I0cPOcGAZ9Wv7QgFFjGXtWwWEvSi\",\"priority\":\"high\",\"data\":{\"title\":\"Some Title\",\"message\":\"Hi\"}}}">>**
and response from FCM is:
**{{"HTTP/1.1",400,"Bad Request"},[{"cache-control","private, max-age=0"},{"date","Thu, 03 Sep 2020 08:57:56 GMT"},{"accept-ranges","none"},{"server","GSE"},{"vary","Accept-Encoding"},{"content-length","3"},{"content-type","text/plain; charset=UTF-8"},{"expires","Thu, 03 Sep 2020 08:57:56 GMT"},{"x-content-type-options","nosniff"},{"x-frame-options","SAMEORIGIN"},{"content-security-policy","frame-ancestors 'self'"},{"x-xss-protection","1; mode=block"},{"alt-svc","h3-29=\":443\"; ma=2592000,h3-27=\":443\"; ma=2592000,h3-T051=\":443\"; ma=2592000,h3-T050=\":443\"; ma=2592000,h3-Q050=\":443\"; ma=2592000,h3-Q046=\":443\"; ma=2592000,h3-Q043=\":443\"; ma=2592000,quic=\":443\"; ma=2592000; v=\"46,43\""}],"to\n"}**
Please suggest what I am doing wrong or missing something. I am calling this method from ejabberd hook which is **offline_message_hook**.

Answer 1

After spending 4 hours I found my mistake. I just remove the payload tag from the request and it worked for me. My updated request is:
post_offline_message(From, To, Body) ->
K = "FCM key",
U = "https://fcm.googleapis.com/fcm/send",
P = #{
to => <<"rid">>,
priority => <<"high">>,
data => #{
<<"title">> => <<"Some Title">>,
<<"message">> => <<"Hi">>
}
},
EP = jiffy:encode(P),
?INFO_MSG("EP data: ~p", [EP]),
httpc:set_options([{keep_alive_timeout, 0}]),
{_, Resp} = httpc:request(post, {U, [{"Authorization", "key=" ++ K}], "application/json", EP}, [], []),
?INFO_MSG("FCM response: ~p", [Resp]).