How would you add menthol crystals into a crope.

So I have a jar on menthol crystals that I want to add to my proraso white tub, I’ve done it before with a tub of the green, melted in microwave with a little water but I also used a bunch of fragrance oil to make the scent stringer after everything was said and done and mixed in I noticed weeks later that the soap had a gummy/sticky feel to it and would stick to thinks like dough, sometimes so much so that a giant glob would stick to my brush, it still works fine but just wasn’t pretty, I have my suspicions it was the fragrance oil and had just put too much adulterants for the soap to hold its normal form, but I figured might as well ask if anyone else has experienced this. Or for those who have added menthol crystals to their soap what method have you used. IV read a few things suggesting everything from just mixing whole crystals in to melting them with a spoon and lighter although that last one sounds a little…crack head ish but I get why, menthol liquefies at higher temps.