This control is always disabled when you open the Global Hotkeys tab-page. Only after having checked the Link active checkbox this control becomes enabled:
This edit-field allows you to insert, modify or remove a hotkey for the PA-RUN Shortcut Link selected in the Dropdown List, for example:
To REMOVE an existing hotkey: Place the caret into the Hotkey edit-field and hit the BACKSPACE key on your keyboard.
See also: PA-RUN ShellLink Shortcuts