Explanation
https://docs.microsoft.com/en-us/azure/active-directory-domain-services/overview Azure Active Directory Domain Services (Azure AD DS) provides managed domain services such as domain join, group policy, lightweight directory access protocol (LDAP), and Kerberos/NTLM authentication Azure AD Domain Services (Azure AD DS) - This one could work since AAD DS will bring in the existing accounts from Azure AD which in turn are synchronised from on-premise AD over AD connect. However, you would probably need to reconfigure the app and update the LDAP connection Azure Active Directory (Azure AD) supports LDAP Authentication via Azure AD Domain Services (AD DS).https://docs.microsoft.com/en-us/azure/active-directory/fundamentals/auth-ldap
https://docs.microsoft.com/en-us/azure/active-directory-domain-services/synchronization