You can use the following code snippet and follow the steps from the provided gif file:
public RadForm1()
{
InitializeComponent();
DataTable dt = new DataTable();
dt.Columns.Add("Id");
for (int i = 0; i < 100; i++)
{
dt.Rows.Add(i);
}
this.radMultiColumnComboBox1.DisplayMember = "Id";
this.radMultiColumnComboBox1.ValueMember = "Id";
this.radMultiColumnComboBox1.DataSource = dt;
this.radMultiColumnComboBox1.SelectedIndex = -1;
this.radMultiColumnComboBox1.SelectedIndexChanged += radMultiColumnComboBox1_SelectedIndexChanged;
}
private void radMultiColumnComboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
RadMessageBox.Show("You just selected index " + this.radMultiColumnComboBox1.SelectedIndex);
}