First, identify the code of the key with « xev » (it is an events catcher)
$ xev
[...]
KeyPress event, serial 48, synthetic NO, window 0x3c00001,
root 0x163, subw 0x0, time 280713293, (261,623), root:(263,648),
state 0x0, keycode 116 (keysym 0xff54, Down), same_screen YES,
XLookupString gives 0 bytes:
XmbLookupString gives 0 bytes:
XFilterEvent returns: False
In this example, the keycode is 116
Use this keycode to enable the repetition on this key with the command « set »
$ set r 116
save this command in .xsessionrc or .xinitrc to keep settings after a restart