Drop-down is browser side, so while technically you can add more attributes to URL, and your Controller can pick these up, you would either had to build a JS script that appends to href value, or you would have to change A tag to a Form submission:
Code:
<from action="<?= site_url("").'User/addsubjects/'.$row['subcode'] ?>" method="POST">
<select name="dropdown">...</select>
<input type="text" name="subcode">
<button>Add</button>
PHP Code:
// Controller
public function addsubject()
{
$dropdown = $this->input->post('dropdown');
$subcode = $this->input->post('subcode');
}
Also, a quick note, to make code more readable, you should add all URL attributes inside site_url function like this
PHP Code:
site_url('User/addsubjects/'.$row['subcode']);
Both ways technically work, but this is tidier and intended way.