curl --request GET \
  --url https://api-m.sandbox.paypal.com/v1/commerce/billing/customers/{external_customer_id}/portal_url \
  --header 'Authorization: Bearer <token>'

{
  "url": "https://www.paypal.com/usage-billing/customer-portal/eyJ1dWlkIjoiNDQ4NmI3NGYtODM5ZC00ZTk2LWEyMTUtNDY3ZDYxMzgwNzFjIiwiZXhwaXJlc19pbiI6MzYwMH0=",
  "expires_in": 3600
}

{
  "name": "INVALID_REQUEST",
  "message": "Request is not well-formed, syntactically incorrect, or violates schema.",
  "debug_id": "dc99e3955cdd8",
  "details": [
    {
      "field": "/email",
      "value": "abc",
      "location": "body",
      "issue": "INVALID_PARAMETER_VALUE",
      "description": "The value of a field does not conform to the expected format."
    }
  ]
}

{
  "name": "NOT_AUTHORIZED",
  "message": "Authorization failed due to insufficient permissions.",
  "debug_id": "565f78f101498",
  "details": [
    {
      "issue": "PERMISSION_DENIED",
      "description": "You do not have permission to access or perform operations on this resource."
    }
  ]
}

{
  "name": "UNPROCESSABLE_ENTITY",
  "message": "The requested action could not be performed, semantically incorrect, or failed business validation.",
  "debug_id": "90957fca61718",
  "details": [
    {
      "field": "transaction_id",
      "issue": "RESOURCE_ALREADY_EXISTS",
      "description": "The resource identified by `field` already exists."
    }
  ]
}

{
  "name": "INTERNAL_SERVER_ERROR",
  "message": "An internal server error has occurred.",
  "debug_id": "90957fca61718"
}