code: func (s *IssueService) AddWatcher(issueID string, userName string) (*Response, error) {
  	apiEndPoint := fmt.Sprintf("rest/api/2/issue/%s/watchers", issueID)
  
  	req, err := s.client.NewRequest("POST", apiEndPoint, userName)
  	if err != nil {
  		return nil, err
  	}
  
  	resp, err := s.client.Do(req, nil)
  	if err != nil {
  		err = NewJiraError(resp, err)
  	}
  
  	return resp, err
  }
docs: // AddWatcher adds watcher to the given issue
  //
  // JIRA API docs: https://docs.atlassian.com/software/jira/docs/api/REST/latest/#api/2/issue-addWatcher