Navigate to Account Settings > Email Settings:
Yahoo Settings: Yahoo requires this setting to be turned on, in order to sync their email accounts with outside applications:
Scroll down until you see "Allow apps that use less secure sign in"
Turn on "Allow apps that use less secure sign in":
If your Account Security page is different from the one above, you may have a Yahoo Business account, in which case you'd need to create an "App password" for 17hats by following this guide: https://help.yahoo.com/kb/new-mail-for-desktop/generate-third-party-passwords-sln15241.html
Use the dropdown to select "Yahoo" and connect your email
Select "Other" from the dropdown and enter these settings, along with your email account's username/password: