Use your internet account with other macOS apps
When you sign in to some websites for the first time, Safari asks if you want to use your account with apps such as Mail, Messages, and Calendar.
To have macOS set up your apps with this account, click one of these buttons:
Add Account: Choose which apps to use the account with, and open them now. (This button is shown for only some accounts.)
Allow: Let other apps use this account. (This button is shown for only some accounts.)
Not Now: Add your account to apps later.
No matter which button you click now, later you can add internet accounts, see accounts you already added, and choose which apps to use each account with. Choose Apple menu 
in the sidebar. (You may need to scroll down.) You don’t need to set up your account separately in each app.