Fixings I need For Medallion Necklace

[ZeusMC]

I'm afraid I know nothing about jewelery making.

I have a medallion like one in screenshot. I want to attach a waxed cord of other type that might be suitable? I'm after a green colured one. If I just attach the cord to a ring, which is fitted to the medallion, then the type of clasp joining the cord, this would just work its way around to where the cord is threaded through the jump ring attached to the medallion.
What type of fixings do I need to get, so can cut the cord to the length I want, joining the cord at both ends and prevent the cord working itself to the medallion? Cord can be in different thickness choice of clasp I think would need to take that into account.
Cheers.