Follow the standard Tableau Server installation steps for your server type.
In the Tableau Server Settings, use the Tableau Server URL you see when viewing a dashboard (IE: "https://10ay.online.tableau.com"). For the Username/Password, you will need a full administrator of the account.
Select "Custom" as the Tableau Server Site and in the manual entry field, type in the site name. (This will also be in the url when viewing a dashboard)
Dashboards loaded in this setup will still have a login box in the middle of the screen when viewing a dashboard for the first time.
Since Tableau Online does not have Trusted Tickets, you will need to implement SAML to have a single sign on system. See the SAML install instructions for more information on setting this up in the portal.
Tableau has excellent documentation on connecting the Tableau Online portion to SAML systems such as Azure AD, OneLogin, PingOne, and Okta: https://onlinehelp.tableau.com/current/online/en-us/saml_config_site.htm
If you do opt for using SAML, follow the following instructions to setup correctly.
At the bottom of the Settings->Authentication page, make sure to select "Single Sign-on with SAML" as the "Default authentication". You will also want to select "Authenticate using an inline frame".
Any users that will be using the portal should be setup to have their "Authentication Type" setup as "Single sign-on with SAML" (See "Users" section).
WARNING: Don't change the Portal's Service User to SAML, this will break the connection to Tableau Server