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