Hello. How can I find the ID of the management server, maybe called FQID?

There is EnvironmentManager.Instance.MasterSite; ref. https://doc.developer.milestonesys.com/html/index.html?base=miphelp/class_video_o_s_1_1_platform_1_1_environment_manager.html&tree=tree_search.html?search=mastersite

It is possible to have multiple sites if using Milestone Federated Architecture (MFA). This is perhaps a special case but if you want to have your integration ready for MFA you should consider the possibility.

var site = EnvironmentManager.Instance.GetSiteItem(EnvironmentManager.Instance.MasterSite);
ShowWithChildren(site);
 
private void ShowWithChildren(Item item)
{
	if (item.FQID.Kind == Kind.Server)
	{
		label1.Text += item.FQID.ToString() + Environment.NewLine;
		foreach (var item2 in item.GetChildren())
		{
			ShowWithChildren(item2);
		}
	}
}