It is not possible to create a new FTP user with '-' symbol in its name. On the attempt to do so in CCP > Websites > example.com > FTP Access > Users > Add New User, the following error appears:
Incorrect or empty field value(s). Please retype info in the highlighted fields. Error: Login name can contain Latin letters (uppercase and lowercase), digits and ! # * ( ) - + = symbols.
Impossibility of '-' symbol usage in FTP Login Name is caused by software-related issue #POA-30719.
The fix is planned to be included in one of future product updates.
Contact your Technical Account Manager or Pooled Technical Associate Team (email@example.com) in order to trace the current status of #POA-30719.