If you're using melt and pour soap, let it set a few minutes until it has just started to harden on the top - until it has a hard enough "skin" to support the rope. Place the rope and give the soap & rope a light spray with some rubbing alcohol. Then fill the rest of the mold with warm (not too hot) re-melted soap.
As with the 3D mold, let it sit until it's properly hardened.
