You can create pop-up subscription forms with GiantCampaign's form editor. They can be triggered by timing, a percentage of your website's scroll, exit intent, or a button click.

Go to the "Lists" tab to create a pop-up form. Then, select "Forms" and click on a button to create a form, which will open the form editor.
First, select the connected list and the form format (in this case, a pop-up form).

Then, select a template and begin editing your form.