Can't bind keys to mouse

[Zezituh]

Hello, don't know if this is a bug or not but, as in other games, i like to bind some keys to mouse. However, in this game, i can't do this. Already tryed a few times and it never let me do it. I don't know if this is normal or anyone else has a problem like me, but this is really annoying.

 

Hope it is possible to get a look into it.