This isn’t a limitation of Webflow, it’s a limitation of Convertkit. You need to ask them, directly, how to solve this for your specific needs.
That said… if you don’t want to use a third party like Zapier or Make (Integromat) then just submit a Convertkit form - as shown in your linked tutorial - with a Convertkit custom tag. On CK’s side, simply setup an automation to add that to a tag.
CK has a help forum, if you’re not understanding what I just mentioned, it may benefit you to ask your question on it.