Default specific option checked in ParentalManyToManyField wagtail, using django-form

Question

I am working with a multi-site wagtail project. And I want an event class which contains ParentalManyToManyField field called sites. The idea is that each event will be associated with multiple sites not necessarily all.

Two sites X and Y have lots of events and most of them are common. So I want to make default checkbox selected for two of the sites while resting unchecked in event content_panels.

My event class is as follows.

from django import forms
from wagtail.core.models import Orderable, Page, Site
from modelcluster.fields import ParentalKey, ParentalManyToManyField

class Event(Page):
    ....
    sites = ParentalManyToManyField(Site)
    ...
Event.content_panels = [
    ...
    FieldPanel('sites', widget=forms.CheckboxSelectMultiple),
    ...
]

I found following code somewhere and tried it but it makes all options checked.

 Event.content_panels = [
    ...   
 FieldPanel('sites', widget=forms.CheckboxSelectMultiple(attrs={"checked":""})) 

I want just two specific options to be checked and rest unchecked by default.