SF_10.1, SF_10.2, SF_11.0, SF_11.1, SF_11.2, SF_12.0, SF_12.1, SF_12.2, SF_13.0, SF_13.1, SF_13.2, SF_13.3, SF_14.0, SF_14.1, SF_14.2, SF_14.3 - https://docs.sitefinity.com/administration-set-password-requirements

ChangePassword.cs

using System;
using System.Configuration;
using Telerik.Sitefinity.Security;
using Telerik.Sitefinity.Utilities.TypeConverters;

namespace SitefinityWebApp
{
    public class ChangePassword
    {
        public void RecoverPasswordCustomMembershiProvider(string MembershipProviderName)
        {
            UserManager userManager = UserManager.GetManager();
            var recoveryMailAddress = userManager.RecoveryMailAddress;

            Type MembershipProvider = TypeResolutionService.ResolveType(MembershipProviderName);

            if (String.IsNullOrEmpty(recoveryMailAddress))
            {

                recoveryMailAddress = ConfigurationManager.AppSettings[MembershipProvider + "_RecoveryMailAddress"];

                if (String.IsNullOrEmpty(recoveryMailAddress))

                    recoveryMailAddress = string.Empty;

            }
        }
    }
}