How to add a cosmetic thread to this part?

Here is the way to do it:

  1. Step 1:

    1. Go to the appearances tab on the right and find the screw thread appearance.

  2. Step 2:

    2. Drag n drop the appearance onto the surface

  3. Step 3:

    3. Edit the appearance and check advanced and go to mapping. Then adjust the width of the texture to get the result you want. Good luck.