When importing an animation to encode

Well, I guess I can help someone new... the setting is right here ... hmmm, no, maybe... no, not there either. Ummm. I am *sure* there was a setting....  (resorts to Google search)... ummm.

I cannot find it either (blush)! Very sorry for the false lead. I have gone through it all, and no, I cannot see it either. 

So only solution I know of is to create a solid color rectangle, import as a puppet, and place in the scene behind the character. I think this is better anyway as otherwise you would have to set the setting each time you exported.

Here I imported the rectangle into the project then added it to the scene behind the character.

But it was too small...

So I went to the Transform behavior and changed the Scale value to something big (default is 100%). I picked 1000% because it was easy to type. Also because I can pan the camera around without running off the edge of the background.

Sorry again for wasting your time with the wrong information before!

Byut very very strange. I thought I had used it once, but clearly cannot have (unless they removed it as a feature, which seems very unlikely).

In CH it is a transparent background (not actually white). You can change it using the little square icon at the bottom right of the scene panel (yours will be white). You can click and cycle through white, black, and checkerboard.

When it reaches Adobe Media Encoder, for MPEG generation there is a setting which can specify the color to insert for transparent. .mp4 files do not support transparency. It defaults to black. So you can update it there.

What is more normal is to create background artwork, load it as a separate puppet, then put it in the scene behind the real character. Then you can control the background color to whatever you prefer (light grey, green-screen green for chroma keying, etc).