I don't know how to add the change reason field in the admin form.
I have followed the instructions adding SIMPLE_HISTORY_HISTORY_CHANGE_REASON_USE_TEXT_FIELD=True to the settings.py file.

I also tried defining a model like this.

from django import forms
from .models import Identity

class IdentityForm(forms.ModelForm):
  class Meta:
    model = Identity
    fields = ['type', 'country', '_change_reason'] #Also tried with 'history_change_reason'   

But I got an error saying that the field _change_reason couldn't be found. Unknown field(s) (change_reason) specified for Identity