Hi ibba, yes you'll want to make the details to come from the main large surface like for example trimming the large piece. There are some examples here:
However, when I examine your piece closely it appears that it is in the shape you want already, the detail is actually already following the same curvature as the main wall.
Maybe it's an issue where you need to increase the density of your polygon mesh export. The slider bar in the mesh options dialog controls the angle parameter, that dices up polygons based on a maximum allowed angle between vertex normals.
However, shapes that are only very slightly curved will not be subdivided very much by that parameter alone, to bring out details on shallowly curved areas you'll also need to use the "Divide larger than" parameter in addition to angle. You put in a distance value in "Divide larger than" and it will then dice up your model more finely like this:
