you can change this in your .ini files
Bind hold down to get ads (if this doesn't work look below):
setbind xxx "Ironsights | onrelease Ironsights"
the above button can break up at times, so make sure you have another key bound to regular ironsights that you can press to fix it when it goes wrong:
setbind xxx Ironsights
If binding this way doesn't work try manually editing your ROInput.ini make the changes under [ROGame.ROPlayerInput] and NOT under [Engine.PlayerInput]
example:
Bindings=(Name="Ironsights",Command="Ironsights",Control=False,Shift=False,Alt=False,bIgnoreCtrl=False,bIgnoreShift=False,bIgnoreAlt=False)
Bindings=(Name="RightMouseButton",Command="Ironsights | onrelease Ironsights",Control=False,Shift=False,Alt=False,bIgnoreCtrl=False,bIgnoreShift=False,bIgnoreAlt=False)