When you open the WordPress app for the first time, it asks for your WordPress.com username (or email address) and password.
WordPress.com Account or Site
To log in to a WordPress.com account or site, you can use your WordPress.com username (or email address) and password. Those are the same login details you use to log in to WordPress.com in a web browser.
If you need, you can reset your WordPress.com password.
To log in to a self-hosted WordPress (not hosted on WordPress.com), you can select the Enter your existing site address link at the bottom of the login page.
This adds another field to the form where you can enter your site’s address (URL).
If you need to reset your password, you can click “Lost your password?” at the bottom of the login form you use to log into your site in a web browser.
You can also use the WordPress guide to Resetting Your Password
for more options.