Looks like my Moodle needs more Moodle. I really fail to see how I can alter that when I can't find where to change the addElement that creates the textfield, be it using jquery or anything else...
Creating the Priority select control was "easy": inside create.php file indicated by Willian I added "$mform->addElement('select','eventpriority', get_string('eventpriority', 'calendar'), ['Baixa','Média','Alta']);" between the Event name field and Event time start field. But that's as far as I could go so far, understanding how to use "$mform->addElement" to include a new element on the form. No idea on how to change the addElement.
BTW, for some reason the link in your post seems broken - I need to copy and paste it on another tab to go there... but anyway, I decided to check that page from top to bottom. "This tutorial is for advanced Moodle devs" - I'm so not there yet =p but let's see if I can take something from there.
Find something like Step 1 mentions on moodle/calendar/view.php - looks like "$renderer = $PAGE->get_renderer('core_calendar');" is an equivalent to the example on that page "$output = $PAGE->get_renderer('core_group');" - but there's no equivalent to "echo $output->render($renderable)". Also, I fail to see an equivalent to $renderable. Also no equivalent for the next public function "render_index_page".
But there are 4 "render_something" on view.php: 3 of those inside a condition that considers day, upcoming or month view. Bunch of variables here - $data, $template, $calendar and $view - most of them linked to the calendar_get_view(), which seems to be where the "new event" button would be created, which is described (defined? I always fail at these concepts of programing - one language will call it this way, another will call the same thing something else...) on calendar\lib.php file at line 3059.
But the only reference for "New event button" is before that, at line 2428. And all it does is check if the user can add event. This function appears 6 times in all files: definition at calendar\lib.php, calendar\managesubscriptions.php, 3 exporters (day, upcoming and month) and the footer. No sign of the button being created - so that trail of thought dies here.
Trying something else - the modal_event_form.mustache, as mentioned in a previous post, only mentions the footer of the modal and the save button (which apparently can't be translated), so that's a dead end.
The AMD file modal_event_form.js... at line 395 begins "Send the form data to the server to create or update an event" - well, seems like this is where I need to mess around if Priority control is going to be of use and not just cosmetic, but that's all.