I know this is 5 months late and you may not be back to see this, but here are some pointers for those who still look here....
What format is your 3D model? (What did you make it with?) - it needs to be in a Sims 4 package format for sims 4 studio to be able to read it.
As of November 2020, Sims 4 does not yet have functional vehicles. The only thing that they can currently ride is a bike.
There are many object tutorials in the wiki that tell you how to do the steps below. http://simswiki.info/wiki.php?title...bject_Tutorials
This is just a fast list, not detailed info but it leads you in the correct direction. (Mainly I am lazy and don't have the time right now to write complete tutorials on how to covert 3d objects to Sims4). :D Modding is not a black box that you stick something in one side and it magically works out the other side. You have to do some homework.
1: Using Sims 4 Studio, Find a suitable sims 4 object like a bike or a large statue and clone that. Caveat- The bicycle's foot pedal motions are weird for a motorcycle but I don't have the skill set to tell you how to disable pedaling. Also, the bike is ridden in parks and on sidewalks - not good for motorcycles. So, maybe just make a large-ish object for now? (2 footprints) (recommended for now unless you know how to create new animations.)
2: Using Sims 4 Studio, Export the LODS and texture maps to your working folder.
3: Open Blender and Load the Sims4 Studio mesh then load your 3d mesh as a wavefront object. Get the UV mapping for each part. lay the UV mapped parts out nice and neat onto the Sims4 studio mesh places. You will need to copy any vertice values from the game object over to your Harley. Objects don't have that many different number values but clothing has a ton of them. They need to be correct if the object moves at all.
4: make your textures - test them in Blender/Milkshape - make sure the textures line up correctly. If they don't, you need to fix the UV maps.
5: Import your LODS and textures back into your package using Sims4 studio.
TEST and hopefully do not crash your game! Good luck!