MouseApp is under development. As far as I know, it doesn't allow yet to create a Mastodon identity. Create your Mastodon account from your computer (your username is your email address), answer the confirmation email. Then you will be able to log in with MouseApp.
@cedesguin I downloaded it, but it was the first app that asked me for my email and password, rather than taking me to the instance for authorization. I declined, not knowing if that was a security issue.
Anyone know why #MouseApp handles this login differently from all the others?