You can change knack's default placeholders in any field using the following code
/* Change text in Knack's placeholders */
$(document).on('knack-view-render.view_30', function(event, view) {
$('#last.input').attr("placeholder","Surname");
$('#first.input').attr("placeholder","Name");
$('#field_31.input').attr("placeholder","your country's phone number format");
});
see the screenshot here:
change my view_30 to the form view of your choice.
Thank you