+-------------------------------+
|  id  |    name    |   code    |
---------------------------------
|  1   | Employee 1 |    A1     |
|  2   | Employee 2 |    A2     |
| ...  |    ...     |   ...     |
+-------------------------------+

SELECT name FROM employee WHERE code LIKE A1

$this->crud->addFilter([
        'type' => 'select2',
        'name' => 'branch_code',
        'label' => 'Filter',
    ],
    function() {
        return Employee::select('code')->distinct()->get()->pluck('code')->toArray();
    },
    function($value) {
        $this->crud->addClause('where', 'code', $value);
    });