correct colorspace for alpha to key+fill on decklink

Hello All,

What is the correct colorspace for sending alpha to a decklink card or
ultrastudio?

Using OBS output I can see key + fill split out as expected.

Trying this pipeline I get fill video duplicated to both outputs on the
ultrastudio
src video is available from
https://www.videezy.com/abstract/50834-red-energy-wire-mesh-ball-with-alpha-channel

gst-launch-1.0 \
filesrc location=/home/video/200607_RedEnergyBallIsolateAlpha.mov !
decodebin ! videoconvert \
! videoscale ! video/x-raw,width=1920,height=1080,framerate=30/1 \
! decklinkvideosink device-number=0 mode=1080p30

I have tried setting profile as 1-4 and also setting the format  as BGRA ||
ARGB but this gives an error
could not link videoscale0 to decklinkvideosink0 can't handle caps BGRA

However the formats list include
video/x-raw
                  width: 1920
                 height: 1080
        pixel-aspect-ratio: 1/1
         interlace-mode: progressive
              framerate: [ 0/1, 2147483647/1 ]
                 format: BGRA

The file source report is
/GstPipeline:pipeline0/GstDecodeBin:decodebin0/avdec_prores:avdec_prores0.GstPad:src:
caps = video/x-raw, format=(string)A444_10LE, width=(int)3840,
height=(int)2160, interlace-mode=(string)progressive,
pixel-aspect-ratio=(fraction)1/1, colorimetry=(string)bt709,
framerate=(fraction)30/1

Any thoughts would be helpful

Thanks
Art Vanderhoffer