If the value must be in a specific format, then it needs to be in a specific format.
At a guess, the Registry entry needs to be a GUID value although a GUID is normally displayed as
########-####-####-####-############
rather than
#########-####-####-####-###########
If so, then it will only accept a GUID value: which is not a "free text" field.
Looks to me like you are trying to hack some WoW system?