ODK Central API POST query for creating draft, using already published definition

1. What is the issue? Please be detailed.

I'm trying to create a draft of an existing form, for uploading attachments, without modifying the xlsx definition, as stated in the API docs:
Additionally, however, you may POST with no Content-Type and an empty body to create a Draft Form with a copy of the definition (XML, XLS, etc) that is already published, if there is one.

This is the code I've tried

curl -u username:password \ -X --include --request POST \ --header "X-XlsForm-FormId-Fallback: {formId} \ https://myodkserver.org:{portnumber}/v1/projects/3/forms/{formId}/draft

and I get this error message:
{"message":"Required parameter formId missing.","code":400.2,"details":{"field":"formId"}}

How can I make the "no Content-Type and an empty body" POST? Thanks in advance