Add additional metafields based on value of select box

Create your metaboxes as you normally would, they will be visible by default. Then use js to hide the ones you dont want initially visible. Your code would look something like below. The example is for a checkbox, but you will get the idea how it works.

if ( $('input#_cmb_metaboxid_1').is(':checked') ) {
        $('#metabox_id_2').show();
    }
    else {
        $('#metabox_id_2').hide();
    }

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