There's no specific configuration needed when making a Dash webapp public.
Please verify that the webapp was added to the list of public webapps in Administration > Settings > Login (LDAP, SSO) & Security. The webapp identifier should be in the format PROJECTKEY.webappid, for example, MY_PROJECT.ocMlc4y.
Additionally, please make sure that you restarted the webapp backend after adding it to the above list, and that the webapp backend is currently running.
Please also make sure that you're accessing the webapp at the correct URL. It should be in the format http(s)://DSS_BASE_URL/public-webapps/PROJECTKEY/webappid/
Finally, please let me know what happens when you try to access the webapp. What error message are you getting?