Hi,

From the question; the requirement looks like you have to add a To-Do activity in the Subscription(sale.subscription) 90 days before the expiry date(In subscription, there is no default field for expiry date. But, there is a field End Date(date)).

For example, I am using the Date of Next Invoice(recurring_next_date) for calculation. If you are using any other date field replace it in the python code given below. Also provide the correct id of the To Do activity type to activity_type_id(here, 4 is given). Since the date is not changing I am using a scheduled action instead of an automated action.

Model: sale.subscription
Execute Every: 1 Days
Number of Calls: -1

license_expiry_date = datetime.date.today() + dateutil.relativedelta.relativedelta(days=90)
subscriptions = model.sudo().search([('recurring_next_date', '=', license_expiry_date)])
for subscription in subscriptions:
  activity = {
    'res_id': subscription.id,
    'res_model_id': env['ir.model'].search([('model', '=', 'sale.subscription')]).id,
    'user_id': subscription.user_id.id,
    'summary': 'Expiry Notification',
    'note': 'The subscription will be expired in 90 days!',
    'activity_type_id': 4,
    'date_deadline': datetime.date.today(),
  }
  env['mail.activity'].create(activity)

Regards