OMG. This is the "helpful" information I finally found: "Third-party email apps (that do not use our {TKNPRODUCTYAHOONAME} branded sign-in page) require you to enter a single password for login credentials. To access your {TKNPRODUCTMAILNAME} account on these apps, you'll need to generate and use an app password. An app password is a randomly generated code that gives a non-{TKNPRODUCTYAHOONAME} app permission to access your {TKNPRODUCTYAHOONAME} account. You'll only need to provide this code once to sign in to your 3rd party email app."

I can't even figure out where the "External Connections" selection is. But I've got a million different layouts and colours to choose from.