kers/events/admin.py

from django.contrib import admin
from django import forms

from .models import Event, EventRegistration


class RegistrationFormSet(forms.BaseInlineFormSet):
    def __init__(self, *args, **kwargs):
        kwargs["initial"] = [
            {"state": EventRegistration.ADMITTED}
        ]
        super().__init__(*args, **kwargs)


class RegistrationInline(admin.TabularInline):
    model = EventRegistration
    extra = 1
    formset = RegistrationFormSet

class EventAdmin(admin.ModelAdmin):
    inlines = [RegistrationInline]

admin.site.register(Event, EventAdmin)