and i want to start with Octane Render and that type of tracking. Like you can see here https://gyazo.com/e51071b7a0f09e2a39f8d0143afcca49 he has a background ( i solved that ) and a flat object, what i want to know is how to make that flat object to add shadows, but, at the same time is "invisible" in th final render, projecting shadows.
If you have R19 (and perhaps as far back as R17) there is a shadowcatcher shader that you apply to a ground object. Depending on your scene you may need to apply a compositing tag with "seen by camera" turned off, but normally shadowcatcher looks after this for you.
You must have a light with shadows turned on and turn on the alpha channel in the render settings. The shadow's alpha shows up in the alpha channel but not the objects alpha itself, and the color of the object's shader is the color of the shadow that shows through.
For just the shadow add a composting tag to the hero object and set seen by camera to off, then just the shadow renders.