The Hero model has the following field.:
added_by = models.ForeignKey(settings.AUTH_USER_MODEL,
        null=True, blank=True, on_delete=models.SET_NULL)
You want the added_by field to be automatically set to current user whenever object is created from admin. You can do this.:
def save_model(self, request, obj, form, change):
    if not obj.pk:
        # Only set added_by during the first save.
        obj.added_by = request.user
    super().save_model(request, obj, form, change)
If instead you wanted to always save the current user, you can do.:
def save_model(self, request, obj, form, change):
    obj.added_by = request.user
    super().save_model(request, obj, form, change)
If you also want to hide the added_by field to not show up on the change form, you can do.:
@admin.register(Hero)
class HeroAdmin(admin.ModelAdmin, ExportCsvMixin):
    ...
    exclude = ['added_by',]