Car auction purchase and register for new V5

[thebullettrain]

I’ve bought a car from BCAA and the underlying seller was Black Horse. I’ve got the green V5 slip, for timing as I want a quick V5, how can I complete it online? I’ve tried using BCAA and Black Horse as the trader but I’ve not had any success: an error and it says unable to find vehicle.

I’ve the same issue if the use the registered company on the V5.