Use Tumblr’s Sign up Page for Login:
Like the web login, Tumblr’s new desktop app for Mac OS X Yosemite defaults to the sign up page if you have logged out. Sure, you can click the login button on the lower left, but why not save a step? Use the sign up page for your Tumblr login.
From the sign up form, enter your email and password. (Note that the Yosemite desktop app for Tumblr does not allow copy/paste for login.) Skip the username. Click the sign up button to log into Tumblr. This shortcut also works on the web login but not the iOS 8 mobile apps.