Hi kboughaba,
I have prepared a quick code snippet which showcases how to achieve the functionality you want.
//In your JS file:

window.onload = function () {
    const scheduler = document.querySelector('smart-scheduler');
    scheduler.addEventListener('editDialogOpen', function (event) {
        const editors = event.detail.editors;
        if (!editors) {
            return;
        }
        const schedulerEvent = event.detail.item,
        dateEndEditor = editors.dateEnd,
        backgroundColorEditor = editors.backgroundColor,
        statusEditor = editors.status;
        let checkbox = document.createElement('smart-check-box');
        checkbox.innerHTML = 'Checkbox label'
        checkbox.addEventListener('click' , () => {
            let statusInput = statusEditor.querySelector('smart-input');
            if(checkbox.checked){
                statusInput.disabled = true;
            }
            else {
                statusInput.disabled = false;
            }
        })
        dateEndEditor.insertAdjacentElement('afterend', checkbox)
    });
};

Also we have two demos regarding customization of the editing window:
https://www.htmlelements.com/demos/scheduler/custom-window/index.htm
https://www.htmlelements.com/demos/scheduler/custom-window-editors/index.htm
Let me know if that works for you!
Please, do not hesitate to contact us if you have any additional questions.
Best regards,
Yavor Dashev
Smart UI Team
https://www.htmlelements.com/