Thrive Themes Pixels
Here's a video showing the 2 ways you can install pixels with Thrive Themes Regular Pages (using one of their 10 themes) and Thrive Architect Landing Pages.
If you are using a theme not from Thrive Themes, you will have to look if they support the insertion of scripts, lots of them do. Otherwise, there are plugins that would allow to do it, or you can manually edit your theme header file.
You can see how in this video how it's done with Thrive.