alpha channel out of Final Cut Express

Does anyone know how to create an alpha channel in Final Cut Express and export the video with this alpha channel? I am trying to pull video without a background into flash and have been able to chroma key out the parts I want, but am not sure how to get it out with an alpha channel.

Thanks!

I got it....

for those that care, in the item proerties of the clip, you must set the alpha to "straight" and also it is best to export using the animation codec with millions+ colors.