the image shown is a n example of a hard surface model being smoothened with a subdivision surface modifier Notice how the edges are so circular?
Hope this helped
You can either add support edges like the other person suggested, or you can use edge creases. Select those two corner edges, go to edge > edge crease and ramp it up to the maximum.
Edit: I've just noticed the car part has thickness. If you smooth the mesh after the extrusion into 3d, you need to crease all 5 corner edges, just a heads up. And if you crease before extrusion into 3d you actually need to crease the two edges inside the mesh pointing into the corner, not the external edges.