How can I make a 2.5D aim offset in C++?

I can’t find any documentation or example on how to make an aim offset for a 2.5D character. I tried getting a world position from DeprojectScreenToWorld, but I can’t find a solution to this. Does anyone know a source or even the function required for this mechanic? Thanks

What exactly would you mean by 2.5D? Would it be something like Octopath Traveler?
Is the character sprite based or 3d with cel shading?

I want to make a side scroller shooter and I need an aim offset. I also need the player to aim at the mouse cursor