Now I found a way to use a GET request to trigger a POST request. I used zapier.

you create a webhook to use as a trigger (it generates a link like this: https://hooks.zapier.com/hooks/catch/xxx/yyy/)

and you configure a new webhook as action. The action can be a POST request, with header, parameters and so on.

I used it to generate a URL to trigger my shelly to open the gate at home.

ask me more if you need more details.