I’m bringing this issue back because no one answered it.

I have the similar issue with StackBlitz SDK in the plugin that I’m working on.
Here is the issue report on their GitHub https://github.com/stackblitz/sdk/issues/28
Here are the StackBlitz docs for the above mentioned feature https://developer.stackblitz.com/platform/api/post-api#example-payload

I discovered that Figma overrides the POST method and instead sends a GET request. This behavior isn't explicitly documented but has been observed when testing with my local server endpoint.

In the following example, the request resolves to a GET method on the server, and the message body is never received:

```

<form id="test" method="post" action="http://localhost:3000/api/generate" target="_blank">

    <input type="hidden" name="message" value="Hello World" />

</form>

<button form="test" type="submit">Submit</button>

```

​​​​@y_toku