#12 Mouse button keyname

[harshash]

I'm creating custom keymap, instead of keys from keyboad. I want to used mouse buttons.
on xterm I know the keyname for mouse buttons eg: <btn4down>, <btn5down>...
Can anyone please tell me what are the keyname for mouse buttons in mrxvt.</btn5down></btn4down>

Mrxvt.macro.Primary+Button3: Str . handle %s \n

I tried RMB, LMB, Button3 amd many more, I can't find the right name for mouse button.