I cannot send webhooks to Slack

Slack only accepts JSON data, as written in their documentation, so you need to format the alert message properly. You can find the example of formatting in the documentation. Instead of a usual alert message 'BTCUSD Greater Than 9000' you should use a key "text" with a value containing the alert message:

{"text":"BTCUSD Greater Than 9000"}

You can find out more about the JSON format in Wikipedia.