How to pass form-data with rest-client - ruby

Question

I am trying to translate a post request I send to a server with Postman to ruby RestClient.
In Postman, I send a (successful) POST request with a URL with params.
In my headers I have one parameter
Under Body I send more parameters under form-data. I tried adding these to the payload, which does not work:
I tried to translate it to the following call with rest-client
url = "https://..."
cookie = "..."
payload = {
payloadParam: "some param",
formDataParam: "form-data param"
}
headers = {
Cookie: cookie
}
Following their docs I then fired the following call:
data = RestClient.post(url, payload, headers)
So this doesn't pass the form-data params correctly. I have a feeling I am supposed to pass these parameters through something other than the payload, but I can't find any docs online explaining how. All I could find is a multipart example of loading an external file, but I don't have an external file.
How do I add form-data params to a POST request with Rest-Client?
My cURL structure looks like this:
curl -X POST \
'https://.../?payloadParam=xxxxxxxx' \
-H 'Accept: */*' \
-H 'Accept-Encoding: gzip, deflate' \
-H 'Cache-Control: no-cache' \
-H 'Connection: keep-alive' \
-H 'Content-Length: 55' \
-H 'Content-Type: application/x-www-form-urlencoded' \
-H 'Cookie: xxxxxxxx' \
-H 'Host: xxxxxxxx.com' \
-H 'Postman-Token: xxxxxxxxxxxxxxx' \
-H 'User-Agent: PostmanRuntime/7.19.0' \
-H 'cache-control: no-cache' \
-d 'formDataParam=xxxxx&secondFormDataParam=yyyyy'