electron/docs/api/structures/post-body.md

PostBody Object

• data (UploadRawData | UploadFile)[] - The post data to be sent to the new window.
• contentType string - The content-type header used for the data. One of application/x-www-form-urlencoded or multipart/form-data. Corresponds to the enctype attribute of the submitted HTML form.
• boundary string (optional) - The boundary used to separate multiple parts of the message. Only valid when contentType is multipart/form-data.

Note that keys starting with -- are not currently supported. For example, this will errantly submit as multipart/form-data when nativeWindowOpen is set to false in webPreferences:

<form
  target="_blank"
  method="POST"
  enctype="application/x-www-form-urlencoded"
  action="https://postman-echo.com/post"
>
  <input type="text" name="--theKey">
  <input type="submit">
</form>