Webflow
Webflow lets you set a form field's Name, so this is the simplest case: name a hidden field to match your field map and the tag fills it by name — no token needed (though the token works too).
Add the tag to your site
In Project Settings → Custom Code, paste the loader into Head Code (or add it as a Custom HTML tag in GTM), then publish.
<script src="https://attr.boggsmtk.com/v1/tag.js?k=YOUR_LICENSE_KEY"></script>
Swap YOUR_LICENSE_KEYfor the client's key from the dashboard.
Add a hidden field for each value
On your form, add a hidden field for each value — either an Embed element containing a raw <input type="hidden">, or a text field you hide. Set the field's Name to exactly the Field Input Namefrom your license's field map (e.g. mtk_ft_source). The tag fills any field with that name, and also pushes the value to the dataLayer under it.
Alternative: paste the [mtk:…]token into the field's Default valueinstead — the tag fills by value, which is handy if a field's name is locked.
Publish
Publish the site. The tag populates the fields on page load and again at submit, so attribution rides along with the lead.