There are couple ways of adding Orthographic Camera to your Lens.
One of the way is to click on the + button in the Objects panel and search for Screen Image and select it.
Now you can see the Orthographic Camera is added to your project automatically with addition of a Screen Image.
You can simply delete the Full Frame Region and now you have an Orthographic Camera in your Lens!
Click on the article to learn more about Screen Transform and Cameras.