You don't add anything to the trailer. Here are is the method I use. This simple drawing represents the two graphics, the truck on the left and the trailer on the right. The crosshair represents the yellow origin (0,0,0) of each mesh that you'll see in the Attachment Maker.
The object is to create a mesh that can be attached to the origin of the truck and that has an attachment point that is far enough to the right so that when the trailer's origin is attached to it, the trailer's pin will be over the trucks fifth wheel.
First I opened the trailer's mesh in the Attachment Maker, selected Add Mesh which generates a yellow Z-Y marker at the trailer's origin. I move it to the left to align beneath the pivot pin and read the value for Y2. In my case is was -6.1. Then close the Attachment Maker without creating a mesh.
Second I opened truck's mesh in the Attachment Maker, selected Add Mesh then move the Z-Y marker from the origin to the right to align with the center of the fifth wheel. Reading the value Y1 as 1.7. Add absolute values of Y1 + Y2 to get 7.8. Place the cursor in the Y Position window and enter 7.8. This moves the the Z-Y marker off to the right. Right click changes the Z-Y marker to green. Note the attachment point window shows its name as a.New. Rename if desired, I didn't. Now save the attachment mesh.
To connect the trailer to the truck, open the truck kuid (Truck International Semi Flying A,<kuid:393523:1408>) for Edit in Explorer and edit the config.txt to make the mesh table. The added text is in red. The result looks like this:
mesh-table
{
default
{
mesh "international_semi_01.im"
auto-create 1
}
default-night
{
mesh "nightview.im"
night-mesh-base "default"
}
attachments
{
mesh "attach_mesh.im"
auto-create 1
effects
{
0
{
kind "attachment"
att "a.New"
default-mesh <kuid:393523:1400>
}
}
}
}
Save and commit.
Bob