using a community as identify provider

Hi 

I am trying to create a SSO between a salesforce community portal and an external app. Here salesforce community portal is the identity provider. when a user logs into the external app, it should redirect to the community page to enter login credentials. How to achieve this using the protocol Open ID connect ?  I tried but didn't found any option in the connected app which will make the thrid party app to login to community page. 

Can some one help on this issue. 

Thank you 

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