The common TT850, their clones and related designs have no lugs to attach a strap directly, nor small lugs (like an old school Metz 45 has) where you could attach a key ring or other strap attachment.
While drilling a hole and attaching some kind of eye bolt would work, that might not even be that stable given the plastic case material - or lead to interesting electrical safety problems.
Rubber band or cable tie hacks are likely not very reliable.
Is there some well known trick, especially given that these flash units are probably more useful as hand held off camera flashes than on camera...
EDIT: Idea is to be able to have a hand free when needed, without needing an assistant, while having a charged and accessorized (in the worst case, with a 20 inch softbox) flash within hands reach immediately. Which I would assume to be a very common need in all kinds of indoor reportage scenarios.