The keyboard character list for typing your email has the dash key. However when you are navigating through the sign in process on a Roku device the keyboard changes for typing your password. They have the underscore character but not the dash “- “ character.
Welcome to community!
The option to use a dash should be available when entering your password through your Roku device. You may need to use your Roku remote and navigate the screen to select special characters.
It may look something like the below picture.
If you continue to have trouble, I would recommend reaching out to Roku Support for further help.