First, you can change the O365 landing page, but you can't specify a URL for the new landing page, it's a dropdown menu. See https://blogs.office.com/2014/09/18/new-office-365-home-page-fast-access-services/ for details.
Second, this setting is a per-user one. An admin cannot change this setting for all or specific users at this time.
As it's said in comments, the solution is to force each user's browser to start on a specific page of your O365 environnement, through GPO. If SSO is not enabled, then they will have to login, and after that O365 will redirect them to the right page (SharePoint site in your case)
But if users explicitly go to portal.office.com, there are no solutions at this time to redirect all of them to a specific page.