by default, if you are using a select type for one of your fields in the order form, the initial select/placeholder will be displayed as ‘———‘
if you want to change that to something more descriptive, you could use the following filter in your theme’s functions.php

add_filter('wppizza_filter_formfields_order', 'myprefix_add_placeholder_to_select');
function myprefix_add_placeholder_to_select($elements){
foreach($elements as $k=>$elm){
if($elm['type']=='select'){	
$elements[$k]['placeholder']='some placeholder';
}
}
return $elements;
}

you can adapt this for non selects too of course, just use

if($elm['key']=='/*selected key you want to set placeholder for*/'){
/*etc*/
}