public class CustomSSRSCredentials : IReportServerCredentials
{
    private string _SSRSUserName;
    private string _SSRSPassWord;
    private string _DomainName;

    public CustomSSRSCredentials(string UserName, string PassWord, string DomainName)
    {
        _SSRSUserName = UserName;
        _SSRSPassWord = PassWord;
        _DomainName = DomainName;
    }

    public System.Security.Principal.WindowsIdentity ImpersonationUser
    {
        get { return null; }
    }

    public ICredentials NetworkCredentials
    {
        get { return new NetworkCredential(_SSRSUserName, _SSRSPassWord, _DomainName); }
    }

    public bool GetFormsCredentials(out Cookie authCookie, out string user,
     out string password, out string authority)
    {
        authCookie = null;
        user = password = authority = null;
        return false;
    }
}

if (!Page.IsPostBack)
{
ReportViewer1.ProcessingMode = ProcessingMode.Remote;
IReportServerCredentials ssrscredentials = new CustomSSRSCredentials("MyUserName", "MyPassword", "ServerName");
ServerReport serverReport = ReportViewer1.ServerReport;
ReportViewer1.ServerReport.ReportServerCredentials = ssrscredentials;
serverReport.ReportServerUrl = new Uri("ReportPathKey");
serverReport.ReportPath = "/Reports/MyReport";
serverReport.Refresh();
}