The SCN wiki will not be available for new content submission starting September 11th 6PM CET until September 14th 6PM CET. Please plan your SCN wiki tasks accordingly.
The okcode field is deactivated when a modal window (the technical name of popup windows) is displayed, so it seems impossible to enter the debugger (using /H okcode).
In common dialogs, the okcode field is available:
But it is not available in modal windows (we see it but we can't enter anything in it):
The solution is to create on the frontend a SAPGUI shortcut (file with extension .SAP), of type System Command and command /H, and drag and drop it from Windows desktop to the SAP modal window. The creation of SAPGUI shortcuts is explained here: SAPGUI shortcuts.
For official information, see Note 118184 - SAPshortcut - debugger start