How to add custom variable in form tag in twig(Laravel)?

How to add custom variable in form tag in twig(Laravel)? For example:

{!! Form::select('slt-mv-driver-name-****[]', $a_slt_user, Input::old('slt-mv-driver-name-1', isset($s_unique_id) ? $a_details_result->data->a_drivers : '') , array('class'=>'form-control multiselect slt-mv-driver-name','id'=>'slt-mv-driver-name-1', 'multiple' => 'multiple')) !!}

In above tag I want to put the value of $abcd dynamically. How can I do that?

via Chebli Mohamed