How to validate form dropdown in codeigniter

Can I know, How  to validate form dropdown in codeigniter.

This is my View File

PHP Code:

<div class="form-group">
                            <label style="color: #E4BC3A; font-size: 18px">Choosing of the runnre</label>
                            <select class="form-control select2" style="width: 100%;" name="runner_id">
                                <option value="<?php echo $row2->runner_id ?>"><?php echo $row2->runner_name?></option>
                                <?php foreach ($supporter_data as $row3){
                                    ?>
                                    <option value="<?php echo $row3->runner_id ?>"><?php echo $row3->runner_name?></option>
                                    <?php
                                } ?>
                            </select>
                        </div> 


This is controller

PHP Code:

   function index()
    {
        if ($this->input->post())
        {
            $this->form_validation->set_rules('runner_id', 'Runner's Name', 'required');
            
        }
        if ($this->form_validation->run() == TRUE)
        {
            $this->Supporter_model->movement_add();
            redirect('supporter/index');

        }
        $data["supporter_data"] = $this->Supporter_model->runner_data();
        $data["movement_data"] = $this->Supporter_model->movement_list();

        $this->load->model("Requisition_model");
        $data["requisition"] = $this->Requisition_model->public_view();
        $data["requisition_done"] = $this->Requisition_model->public_view_job_done();
        //var_dump($data["requisition"]); die();
        $this->load->view('supporter/public', $data);
    }