How to add checkbox in pop-up templates?

Site URL: https://www.buildingpictures.pt/

Hello!
I want to create a pop-up so people can subscribe to my newsletter. But in the pop-up templates available through Squarespace, I can't add a checkbox. I know this is possible in Squarespace forms, but in the template of the pop-up, I can't do it. And I want to use the pop-up because it is flashier. For the company, it is essential to have this checkbox so that people accept the terms and conditions before subscribing to the newsletter. In the European Union, this is mandatory because of the GDPR.
Can you help me?
Is there any way to include this checkbox in the pop-up or even inject the Javascript code? I know the code I can use, but I don't know where I should put it to include this in the pop-up.
I send here the image of the pop-up I make. I would like to include the checkbox that appears in the second image.
I would be very grateful if you could help me.
Thanks,
Building Pictures team

7 minutes ago, buildingpictures said:

I want to create a pop up so people can subscribe to my newsletter [but] I can't add a checkbox. It is essential to have this checkbox so that people accept the terms and conditions before subscribing to the newsletter. In the European Union this is mandatory because of the GDPR.

Contrary to popular belief, a checkbox is not required under the GDPR when you are collecting data that is required for the single purpose stated, in this case collecting the user's email address to send them a newsletter, and nothing else.

This is why the Promotional Popup has been designed this way. It doesn't include options to collect additional [unnecessary] data and so doesn't require a checkbox. You can clearly state the purpose using the caption and this will be sufficient as long as you do not use the email address for any other purpose.

If you were collecting data that was not directly related to the stated use, for example their name or their location, or if you planned to use the data for other purposes, you would need explicit consent for each additional actionable use.

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I have a similar request -- I'd like a checkbox on forms so that the person can give an email to get a freebie, and also check a box to sign up for a newsletter. I believe this is what GDPR wants for freebies. And since a *lot* of people use freebies as a way to build their list, this is a very important feature some of us have been wanting for the last 5 years.