These are Elastic Neck Cords made especially for your badge holders.
These cords are half the cost of chains and lanyards, which is awesome!
Insert the end into the small hole with the metal folded against the cord. When through, make sure the metal piece snags onto the holder, which now it is fully attached.
Here is a look at it in action, notice you can change the orientation of the metal barbed ends to be visible or not visible.
They are 30” in length, but stretch out to much longer, and always come back to 30 inches.
Your badge holder must have the two small holes as seen here to ensure that they are secure.
If you are worried they might come out, here is a test—of me shaking the holder violently(speeded up)—it took a lot of shaking to get just one loose. There is a chance they could come loose, but it’s not high.