This article describes how to pass “username@domain.com” information to server from NetScaler, instead of just the username ( sAMAccountName) while performing LDAP Authentication.
Change the "SSO Name Attribute" in the LDAP profile to User Principal Name (UPN) as shown below:
"Server Logon Name" is what the user enters(sAMAccountName) while logging into the gateway authentication page and "SSO Name Attribute" is what is sent to the backend server(userPrincipalName) for the SSO.