curl --request POST \
  --url https://api.sprucecollective.co.uk/admin/subscriptions/assign \
  --header 'Authorization: <api-key>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "billing_interval": "daily",
  "duration_months": 12,
  "reason": "<string>",
  "site_id": "<string>",
  "tier": "<string>",
  "user_id": "<string>",
  "start_date": "<string>"
}
'

{
  "amount_cents": 123,
  "auto_renew": true,
  "available_credits": 123,
  "billing_interval": "<string>",
  "can_be_cancelled": true,
  "cancellation_reason": "<string>",
  "cancelled_at": "<string>",
  "cooling_off_expiry": "<string>",
  "created_at": "<string>",
  "credits_used_this_period": 123,
  "currency": "<string>",
  "current_period_end": "<string>",
  "current_period_start": "<string>",
  "entry_credits_per_period": 123,
  "id": "<string>",
  "is_active": true,
  "is_cancelled": true,
  "is_in_trial": true,
  "is_paused": true,
  "is_trial_active": true,
  "metadata": {},
  "next_billing_date": "<string>",
  "paused_until": "<string>",
  "payment_method": "<string>",
  "site_id": "<string>",
  "status": "<string>",
  "subscription_start_date": "<string>",
  "tier": "<string>",
  "trial_end_date": "<string>",
  "trial_start_date": "<string>",
  "updated_at": "<string>",
  "user_id": "<string>"
}