Slack JSON Proxy will map your JSON payload to application/x-www-form-urlencoded
update .env file
PROXY_ENDPOINT=https://slack.com/api
PORT=3000
npm start
npm test
every path you request will be mapped to PROXY_ENDPOINT/path
PROXY_ENDPOINT=http://slack.com/api
https://host/api/api.test -> https://slack.com/api/api.test
https://host/auth.test -> https://slack.com/auth.test