Callback function with optgroup

Hello,

Yes, it is possible. You can define a custom function in the file functions.php before adding the callback function in the MB builder. The choice should have the format: value => Label. For example:

function my_custom_callback() {
    return [
        'branda' => 'BrandA',
        'model1' => 'Model1',
        'model2' => 'Model2'
    ];
}

Hello,

For the option group, it isn't possible. But you can try to set the sub-choices like this

function my_custom_callback() {
    return [
        [
            'value' => 'monkeys',
            'label' => 'Monkeys',
        ],
        [
            'value' => 'king_kong',
            'label' => 'King Kong',
            'parent' => 'monkeys',
        ],
        [
            'value' => 'curious_george',
            'label' => 'Curious George',
            'parent' => 'monkeys',
        ],
        [
            'value' => 'donkeys',
            'label' => 'Donkeys',
        ],
        [
            'value' => 'eeyore',
            'label' => 'Eeyore',
            'parent' => 'donkeys',
        ],
        [
            'value' => 'guss',
            'label' => 'Gus',
            'parent' => 'donkeys',
        ],
    ];
}

and set the setting flatten to false. Follow the documentation https://docs.metabox.io/fields/select/#settings

Hello,

You can set the flatten setting by using the custom settings of the field. Please check this screenshot https://imgur.com/kiiEgca
It is noted in the documentation https://docs.metabox.io/extensions/meta-box-builder/#custom-settings