Before sending webhooks to Discord, please read the Developer Documentation. You must format your message properly depending on the action you want to perform. For example, if you want to post a message to the chat, instead of a usual alert message 'BTCUSD Greater Than 9000' you should use the "content" key with a string value containing the alert message (as it is written in the documentation), instead of the usual alert message 'BTCUSD Greater Than 9000.' Look below for an example:
{"content":"BTCUSD Greater Than 9000"}
You can find out more about the JSON format on Wikipedia.