Based on the provided Webex API documentation for "Create a Membership," the correct HTTP method to add a user to a space is POST. The request must be sent to the /v1/memberships endpoint. The JSON payload in the request body requires specific key-value pairs to identify the target space and the user. The roomId key is used to specify the space's identifier ("987654321"). To add the user by their email address, the personEmail key must be used with the value "[email protected]". Finally, to grant moderator privileges, the boolean parameter isModerator must be set to true.

Cisco Webex for Developers - Official API Documentation:

Resource: Create a Membership API Reference

URL: https://developer.webex.com/docs/api/v1/memberships/create-a-membership

Details: This official documentation confirms that creating a membership requires a POST request to the /memberships resource. It lists roomId, personEmail, and isModerator as valid body parameters for the request, matching the structure required by the question.