1) Simple way of tracking after redirection to the "Thank you" page.
In case, if you do not have showing "Payment form(s)" after submission of the bookings (in paid versions of Booking Calendar), and you have defined the "Redirection to the "Thank you" page (instead o showing "Thank you" message) at the Booking > Settings General page in Form section, so then simplest way to add your "tracking code" (like Google Adwords code) inside of your "Thank you" page. Please open your "thank you" page for editing, switch to the "Text view mode" of your content editor (html view mode) and insert your Tracking code, which was provided by Google Adwords or some other tracking service.
2) Other more flexible way of tracking booking form submission is adding code in your functions.php file of your theme.
Firs of all please update your version of Booking Calendar to the update 8.6 or newer. You can request the new update of paid versions of Booking Calendar on this page.
After this add inside of the functions.php file of your theme code similar to this,
(you can check how to edit files in WordPress menu in this article):