Hi @DZwier, I hope that you are well!
Great question, thanks for asking the Community!
I’d like to ask clarifying questions to make sure I understand the behavior:
1. Is the popup form a HubSpot form? Or an external form?
2. If the website hosted on HubSpot or an external website?
3. If it is a HubSpot form, have you checked this article “Troubleshoot HubSpot forms”
4. What is the URL of the website so that we can have access to the HTML code if needed
The more info, screenshots (without sensitive/confidential information), and details you can provide, the better the Community can assist.
I found similar threads that might help you:
- The solution from @elizheleva on this post “Pop Ups are not passing hidden fields”
- The solution from @MatthewShepherd on this post “Pop-up Form within <form> tags are not getting submitted in Hubspot”
I’d like to also invite some of our Top Experts to this conversation: Hi @Mike_Eastwood, @ArtyCele, @TomM2 and @Jnix284 do you have tips to help @DZwier, please?
Also, if anybody else has anything to add and/or share, please feel free to join in the conversation 
Thank you so much 
and have a brilliant day!
Best,
Bérangère