All you would do is insert the embed code in your Webflow site, assuming you have access to the HTML code in that web-builder.
It seems Webflow is no different to say, Wix in that respect and you should be able to embed the form's code.
As the basic level you could still insert a hyperlink to your machform in the pages you're creating.
The only requirement would be standard web hosting such as Siteground or Hostgator, to host Machform on if not using Appnitro's hosted solution. So Machform itself would not be situated "in" the Webflow site, but on another server entirely. If that doesn't suit your client, then it probably isn't possible to do any other way.