Can you show as your full EBS Send call with secrets removed?
Are you posting to similar to the example:
curl -H "Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJleHAiOjE1MDMzNDM5NDcsInVzZXJfaWQiOiIyNzQxOTAxMSIsImNoYW5uZWxfaWQiOiIyNzQxOTAxMSIsInJvbGUiOiJleHRlcm5hbCIsInB1YnN1Yl9wZXJtcyI6eyJzZW5kIjpbIioiXX19.TiDAzrq58XczdymAozwsdVilRkjr9KN8C0pCv7px-FM" \
-H "Client-Id: pxifeyz7vxk9v6yb202nq4cwsnsp1t" \
-H "Content-Type: application/json" \
-d '{"content_type":"application/json", "message":"{\"foo\":\"bar\"}", "targets":["broadcast"]}' \
-X POST https://api.twitch.tv/extensions/message/27419011
Note that ChannelID in your JWT payload needs to casted to a string.
And you need to post to the right channelID in https://api.twitch.tv/extensions/message/27419011
If you post a VALID JWT payload with one channelID to the DIFFERENT channel ID it 204’s in error.