WebForm1.aspx.cs

using DevExpress.DashboardWeb;
using DevExpress.DataAccess.Web;
using System;
using System.Linq;

namespace ASPxDashboard_CustomParameters {
    public partial class WebForm1 : System.Web.UI.Page {
        protected void Page_Load(object sender, EventArgs e) {
            DashboardFileStorage dashboardFileStorage = new DashboardFileStorage("~/App_Data/Dashboards");
            ASPxDashboard1.SetConnectionStringsProvider(new ConfigFileConnectionStringsProvider());
            ASPxDashboard1.SetDashboardStorage(dashboardFileStorage);
            ASPxDashboard1.CustomParameters += ASPxDashboard1_CustomParameters;
        }

        protected void ASPxDashboard1_CustomParameters(object sender, CustomParametersWebEventArgs e) {
            var countryParam = e.Parameters.FirstOrDefault(p => p.Name == "CountryDashboardParameter");
            if (countryParam != null) {
                countryParam.Value = "Brazil";
            }
        }
    }
}