Hi SUJAYS,

Using the article i have created the example for multiple parameter.

Call (Consume) Web API in Code Behind in ASP.Net using C#

Refer below code.

Namespaces

using System.Collections.Generic;
using System.Configuration;
using System.Data.SqlClient;
using System.Web.Http;

Model

public class CustomerModel
{
    /// <summary>
    /// Gets or sets Name.
    /// </summary>
    public string Name { get; set; }

    /// <summary>
    /// Gets or sets City.
    /// </summary>
    public string City { get; set; }
}

ApiController

public class CustomerAPIController : ApiController
{
    [Route("api/CustomerAPI/GetCustomers")]
    [HttpPost]
    public List<Customer> GetCustomers(CustomerModel customer)
    {
        List<Customer> customers = new List<Customer>();
        string constr = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
        using (SqlConnection con = new SqlConnection(constr))
        {
            string query = "SELECT CustomerID,ContactName,City FROM Customers WHERE (ContactName LIKE @Name + '%' OR @Name IS NULL) AND (City = @City OR @City IS NULL)";
            using (SqlCommand cmd = new SqlCommand(query))
            {
                cmd.Connection = con;
                cmd.Parameters.AddWithValue("@Name", !string.IsNullOrEmpty(customer.Name) ? customer.Name : (object)DBNull.Value);
                cmd.Parameters.AddWithValue("@City", !string.IsNullOrEmpty(customer.City) ? customer.City : (object)DBNull.Value);
                con.Open();
                using (SqlDataReader sdr = cmd.ExecuteReader())
                {
                    while (sdr.Read())
                    {
                        customers.Add(new Customer
                        {
                            CustomerID = sdr["CustomerID"].ToString(),
                            ContactName = sdr["ContactName"].ToString(),
                            City = sdr["City"].ToString()
                        });
                    }
                }
                con.Close();
            }
        }

        return customers;
    }

    public class Customer
    {
        public string CustomerID { get; set; }
        public string ContactName { get; set; }
        public string City { get; set; }
    }
}

HTML

Name:<asp:TextBox ID="txtName" runat="server"></asp:TextBox>
<br />
City:<asp:TextBox ID="txtCity" runat="server"></asp:TextBox>
<asp:Button ID="btnSearch" runat="server" Text="Search" OnClick="Search" />
<hr />
<asp:GridView ID="gvCustomers" runat="server" AutoGenerateColumns="false">
    <Columns>
        <asp:BoundField ItemStyle-Width="150px" DataField="CustomerID" HeaderText="CustomerID" />
        <asp:BoundField ItemStyle-Width="150px" DataField="ContactName" HeaderText="Contact Name" />
        <asp:BoundField ItemStyle-Width="150px" DataField="City" HeaderText="City" />
    </Columns>
</asp:GridView>

Namespaces

using System.Net;
using System.Text;
using System.Web.Script.Serialization;

Code

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        this.PopulateGridView();
    }
}

protected void Search(object sender, EventArgs e)
{
    this.PopulateGridView();
}

private void PopulateGridView()
{
    string apiUrl = "http://localhost:26404/api/CustomerAPI";
    object input = new
    {
        Name = txtName.Text.Trim(),
        City= txtCity.Text.Trim()
    };
    string inputJson = (new JavaScriptSerializer()).Serialize(input);
    WebClient client = new WebClient();
    client.Headers["Content-type"] = "application/json";
    client.Encoding = Encoding.UTF8;
    string json = client.UploadString(apiUrl + "/GetCustomers", inputJson);

    gvCustomers.DataSource = (new JavaScriptSerializer()).Deserialize<List<Customer>>(json);
    gvCustomers.DataBind();
}

public class Customer
{
    public string CustomerID { get; set; }
    public string ContactName { get; set; }
    public string City { get; set; }
}

Screenshot