To hide the Sign in with an external account section from the Sign In and Register page of the portal.
Set the following site settings value to false à
Followed by Restart of the portal.
To add any additional JavaScript or HTML, for e.g. we wanted to disable the Email text box on the Register page,
Go to Portals à Content Snippets and create a new code snippet record
The output:-
The helpful post –
http://benitezhere.blogspot.ae/2018/01/hiding-sign-in-with-external-account.html
http://arpitmscrmhunt.blogspot.ae/2017/12/customise-registration-page-in-crm.html
Hope it helps..
Nishant Rana's Weblog 
Hi Nishant thank you for this helpful post but just wanted to know one thing I am trying to excess this portal on the online trial account of the Crm online 365 but can’t see the portal option the account I created was actually having every thing like all modules but still can’t see portal option. Am I missing any thing for configuration etc ?
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this might help hyou https://nishantrana.me/2017/01/09/setting-up-portal-trial-in-dynamics-365/
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