The contact message window (pop up  or modal) is opened via javascript when you push the contact button .

The code is in property.js at line 170

$('#contact_host,#contact_me_long').click(function () {
var booking_id, agent_id;
agent_id = 0;
booking_id = $(this).attr('data-postid');
wpestate_show_contact_owner_form(booking_id, agent_id);

The javascript click event is calling another javascript function that is located in property.js at line 146

function wpestate_show_contact_owner_form(booking_id, agent_id) {

In this function we read the some data (like property owner) and we send it via an ajax call to a php function called wpestate_ajax_show_contact_owner_form(). The php function (located in ajax_functions.php at line 910) then gets the data send via ajax as $_POST  and create the html form  to show the user.

If you want to change/delete/add info this is the place where you need to add your input fields.

After the form is displayed the initial javascript function  ( wpestate_show_contact_owner_form  ) will enable the send button action by calling the function enable_actions_modal_contact(in property.js at line 182)

The send message action is at line 212

$('#submit_mess_front').click(function (event) {
var ajaxurl, subject, booking_from_date, booking_to_date, booking_guest_no, message, nonce, agent_property_id, agent_id;



Here we read the info submited by user like booking dates, message, etc

booking_from_date = $("#booking_from_date").val();
booking_to_date = $("#booking_to_date").val();
booking_guest_no = $("#booking_guest_no").val();

Any fields added will have to be read here and used in the ajax call tha follows

And we send that data via another ajax call to a a php function wpestate_mess_front_end() located in ajax_functions_booking.php at line 880.

In this function we retrieve the data send via ajax (as $_POST vars) , and compose the email that is inserted into inbox system via wpestate_add_to_inbox and we send an email via function wpestate_send_booking_email.

If you added new fields : you will have to retrive the data via $_POST and use those when sending the emails. Or you could just write your new send email function.