I was trying the new Assistant in Fusion to create 3D models and it works quite well, although rather slow. But when I tried similar prompts in AutoCAD, its just gives me the workflow, it doesn't create geometry like in Fusion. Can I enable this somewhere?

No, this behavior is "as designed". This may change in the future, but currently the Autodesk Assistant in AutoCAD 2027 can just analyze the drawing and select objects, it doesn't modify any geometry (like the AA in Fusion) - although he knows how to do that and can give you an exact script to perform the desired modifications.See:https://www.cadforum.cz/en/examples-of-ai-prompts-and-the-challenges-of-using-ai-assistant-tip14969