This looks like the same 3D model you posted yesterday and it isn't exactly a beginner project.
My advise would be the same. Model the spoiler in box-view mode and change to smooth-view mode for checking. You have very many self-intersecting polygons in that model so it is unlikely going to convert into a solid body or surface body.
You are aware that your timeline is turned off because you are working on an imported model ?
That isn't necessarily a bad thing f you are working with T-Splines only (in that case it can actually be quite helpful) but if you are starting to work with surfaces and/or the tools in the Model environment you might want to consider turning the timeline on.
There are several ways this can be "matched". You can convert the trunk mesh to a T-Spline and then into a surface body or solid body. Then, if you spoiler is a solid body you can combine-cut the trunk and the spoiler.
The mesh you downloaded was modeled in a Sub-D mesh modeling software such as Modo, Maya, 3DS Max, Blender etc. This mesh is not the original mesh o the Sub-D control cage it is the already subdivided mesh. That means the original mesh has already be subdivided in the originating software likely using catmul-clark subdivision (or open subdiv).
That mesh/model you imported into Fusion 360. Because it is a quad mesh (no triangles) it can be converted not a T-Spline and then into a solid body or surface body. These sort of meshes are almost always modeled for visual representation, so they are not closed (water tight) so they convert into a surface body.
You can now model your spoiler and then, after you have modeled the T-Spline convert the spoiler into a solid body and use the trunk surface to split that solid body. The split surface then will match the trunk perfectly.