How about something like this:
s ="UserID/ContactNumber"
so = ''
for counter, char in enumerate(s):
if counter == 0:
so = char
elif char.isupper() and not (s[counter - 1].isupper() or s[counter - 1] == "/"):
so += '-' + char
else:
so += char
print(so.lower())
What have I changed from your fragment?