Hey there Sunil! To get the items in a dropdown in the admin, you just need to query from the Collection model and make those your choices. See incomplete code sample below. This will store the collection, allowing you to manipulate and filter the data for page context as needed. Hope that helps!
...
from wagtail.core.models import Collection, Page
class YourPage(Page):
...
# Convert Collections queryset into choices tuple
COLLECTION_CHOICES = ((
c.id,
c.name) for c in Collection.objects.all())
# Add choices tuple to field
collections = models.CharField(max_length=255, choices=COLLECTION_CHOICES)
...