code: func (api *Client) InviteGuestContext(ctx context.Context, teamName, channel, firstName, lastName, emailAddress string) error {
  	values := url.Values{
  		"email":            {emailAddress},
  		"channels":         {channel},
  		"first_name":       {firstName},
  		"last_name":        {lastName},
  		"ultra_restricted": {"1"},
  		"token":            {api.token},
  		"resend":           {"true"},
  		"set_active":       {"true"},
  		"_attempts":        {"1"},
  	}
  
  	err := api.adminRequest(ctx, "invite", teamName, values)
  	if err != nil {
  		return fmt.Errorf("Failed to invite single-channel guest: %s", err)
  	}
  
  	return nil
  }
docs: // InviteGuestContext invites a user to Slack as a single-channel guest with a custom context