I'm not allowed to create this by myself because the difference is the dash only, so I raise it on Meta. I'm not sure if this is the right place to propose for tags.
to avoid confusion errors
I'm not allowed to create this by myself because the difference is the dash only
Looks like the system is doing its job. Specifically, the message you should be getting is
You are attempting to create the tag [codegolf]; however the tag [code-golf] already exists!
I don't see any potential for confusion there.
Synonyms are created when two tags are being used for the same purpose, making searching, categorization, etc. difficult. Since [codegolf] cannot even be created, adding a synonym would be pointless.