Use the Matte shader with camera back plate textures or videos. See Setting a camera Front or Back Plane.

To create a 3D matte:

1. Drag the Matte shader from the Shaders folder in the Asset browser, and drop it onto your model.
2. Select Replace by Type from the contextual menu that appears.
3. In the Matte shader settings, select Highlight Matted Objects to surround the object with a faint outline if you are having trouble viewing the matted objects in your scene.
4. Activate Enable 3D Masking option to apply the 3D Matte shader to the selected object. When Enable 3D Masking is not active, the shader appears as a wireframe.
5. Activate Adjust Alpha to control the video Alpha channel sent either through your video option board or viewed in the Render window.
6. Use the Alpha slider to manipulate the transparency of the Alpha channel. This lets you, for example, make the output slightly transparent so you can pre-visualize characters interacting with live Actors.