Thanks for reaching out in the Community! That's a great question.
When you set up the pop-up sign-up form within your account, part of the setup is to tell it which of your email lists the new sign-ups will be put on. You can find that information by clicking on Sign-Up Forms at the top of your account, clicking on the three vertical dots on the Pop-Up form, and choosing Edit. On the left hand side you will see a selection for "Email Lists". There you will find which list(s) you assigned the new contacts to!
If you have any further questions please feel free to reach out to us here. We are always happy to assist you in any way we can!