I found a way to do it with a custom html embed module on external site builder (webflow). First, create a new CTA using the new CTA builder (toggle away from legacy CTA). I chose the webinar pop up registration template as it has a background and a form. Choose a form and pulblish it.
Next, create another CTA button in the new CTA builder, I chose the sign up now template. Choose link to CTA, then choose the pop up CTA you just published above. See pic below:
Lastly, go to the CTA button you created and click edit and then click embed in the upper right. Copy the embed code and paste it in a custom html embed module on your page builder. Make sure you have the hubspot tracking code installed on the external website, otherwise the pop up won’t work.