To be able to add this functionality to a login page, SSO options must first be created by the admin.
For instructions on this process, see the Help Center article (including links to specific SSO platforms) here.
Go to Design > click Login Pages > select existing login page or click New
Remain in the Details tab
Scroll down to "Single Sign-On Options" > choose which SSO provider(s) to display.
*Note: To select multiple options, hold the CTRL-key of your keyboard and click the multiple options.
The selected SSO option(s) will now be displayed on this login page.