code: func (c *Client) DomainCreate(appIdentity string, hostname string) (*Domain, error) {
  	params := struct {
  		Hostname string `json:"hostname"`
  	}{
  		Hostname: hostname,
  	}
  	var domainRes Domain
  	return &domainRes, c.Post(&domainRes, "/apps/"+appIdentity+"/domains", params)
  }
docs: // Create a new domain.
  //
  // appIdentity is the unique identifier of the Domain's App. hostname is the
  // full hostname.