Reply To: Hide Metabox Field on Custom Frontend Form

I see. Can you try CSS approach, like this:

array(
    'name'  => 'Website Screenshot URL:',
    'desc'  => '',
    'id'    => $prefix . 'website_screenshot',
    'type'  => 'text',
    'class' => 'hidden',
),

And in the Customizer > CSS and add this:

.hidden { display: none; }