How attach mesh hair to model hair

Trying to attach the hair. Been struggling for more than a week now
ctrl+join makes the hair ‘‘dissapear’’
Also parenting with auto weights, distorts the body wildly
In case some of you have time to check it, I’ve attached the blendfile
https://mega.nz/file/scRilTiA#eripMGIqLm4gTC0jcP10xN5D1RTfWe_hkCUo6n4Cn28