Create a branded link

**This endpoint allows you to create a new branded link.** To create the link branding, supply the root domain and, optionally, the subdomain — these go into separate fields in your request body. The root domain should match your FROM email address. If you provide a subdomain, it must be different from the subdomain you used for authenticating your domain. You can submit this request as one of your subusers if you include their ID in the `on-behalf-of` header in the request.