Hi Veronica, are you trying to generate a new live CMS item from the Webflow form, and tie it to a multi-reference field in another collection? The only way I was able to successfully get mine to work was with Webhooks by Zapier. You need to leave the multi-reference field blank in your first zap, then create a second “Webhooks by Zapier” to tie the new live item to the other CMS collection, using the Webflow ID in quotation marks if part of a series.
Testing the API call with Postman saved me from pulling my hair out.
If you want to share any screenshots I might be able to help further.