Be sure that "Fastkeys!" checkbox at lower left of the "Send Keystrokes" window is selected.
Be sure that the hotkey is enabled.
It seems to work fine for me even selecting text in this message edit window.
SHIFT+END (in hotkeys this would be "+{end}" without quotation marks) selects text from cursor position to end of line.
The only other thing I can think of is that (it seems) that I can have more than 2 or 3 dozen hotkeys per hotkey definition file some of the hotkey functions can get flaky. This is the only difficulty I have run into using Hotkeys with Windows 98se.