How to find my web api key ?

Model part

public class Employee
{
public int ID { get; set; }
public string Name { get; set; }
public DateTime JoiningDate { get; set; }
public int Age { get; set; }
}

Controller part

public class EmployeesController : ApiController
{
Employee[] employees = new Employee[]{
new Employee { ID = 1, Name = "Mark", JoiningDate =
DateTime.Parse(DateTime.Today.ToString()), Age = 30 },
new Employee { ID = 2, Name = "Allan", JoiningDate =
DateTime.Parse(DateTime.Today.ToString()), Age = 35 },
new Employee { ID = 3, Name = "Johny", JoiningDate =
DateTime.Parse(DateTime.Today.ToString()), Age = 21 }
};
public IEnumerable GetAllEmployees()
{
return employees;
}
public IHttpActionResult GetEmployee(int id)
{
var employee = employees.FirstOrDefault((p) => p.ID == id);
if (employee == null)
{
return NotFound();
}
return Ok(employee);
}
}

Here I am populating grid using Api data

public void populateWebApiGridView()
{
string URL = "http://localhost:34613/api/Employee";
string urlParameters = "API_KEY";                                       ///////Here I need the key
DataTable dt = new DataTable();
dt.Columns.Add("ID", typeof(int));
dt.Columns.Add("Name", typeof(string));
dt.Columns.Add("JoiningDate", typeof(DateTime));
dt.Columns.Add("Age", typeof(int));
HttpClient client = new HttpClient();
client.BaseAddress = new Uri(URL);
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
HttpResponseMessage response = client.GetAsync(urlParameters).Result;
if (response.IsSuccessStatusCode)
{
var dataObjects = response.Content.ReadAsAsync>().Result;
foreach (var d in dataObjects)
{
dt.Rows.Add(d.ID);
dt.Rows.Add(d.Name);
dt.Rows.Add(d.JoiningDate);
dt.Rows.Add(d.Age);
}
}
else
{
System.Diagnostics.Debug.WriteLine("{ 0} ({1})", (int)response.StatusCode, response.ReasonPhrase);
}
client.Dispose();
grdEmployee.DataSource = dt;
grdEmployee.DataBind();
}

As you have mentioned you have created the API so I believe you must be aware about it fully. What do you mean by Key of the API?

In common practice we usually pass all the secured data in header of the API.

Please provide us the code and the issue you are encountering.