It's called a Sam Browne belt.
You undo the belt strap at the bottom, the shoulder strap (on the same side). Slip into it, then redo-up the belt and shoulder.
A Sam Browne belt has a strap across one shoulder that crosses to the opposite side of the wearer's body at the front and back to support the weight of a pistol or sword.
It keeps the driver/passengers back in their seats, to prevent their heads from hitting the windshield, dash, or steering wheel.
It should have the shoulder and seat belt.
Assuming everything is working right, the strap doesn't lock till you hit something. There is a weight in the base where the strap coils up that locks the strap when you hit something. You can usually test this by jerking the strap and it should lock. Question. Is this for a child seat. There may be a center lap only belt that locks solid. Very often it is tucked down behind seat and you have to look for it.
a belt. or leather strap or is that a snack?
a holster
Spainsh for belt or leather strap
A baldric is a belt worn over one shoulder and across the chest to support a sword or bugle, etc. It is quite similar to the Sam Browne belt worn with certain military uniforms.
Lap and shoulder belt combinations in your car are called safety belts
Forgive me if you have tried this already , but have you moved the seat all the way back ?
putting the strap in the buckle