Help properly rendering to Compositing on VSE

Hi everyone, i’m trying to render an animation, wich i want to do in layers, it’s a fixed camera and only one animated object, most of the scene is fixed.
I know i can render just one frame for the backgorund, and then render the animated object separatedly but i’m having problems with the alpha channel for the latter, i can see the object without background as it should be, but it has no alpha map, so, i can’t use the alpha over modifier on the VSE.
Can anyone point me on the right direction on how to do this ?

Enable transparency in render properties.

in the film tab… thanks JA12