Underline in Persian keyboard

I'm trying to add underline between two word in Persian.

In English , shift + - combination works well and I can type underline character well, like this:

Ask_Ubuntu

But in Persian this keys combination not work. And when I press shift + - instead of underline typing, the letters stretch like this:

اسکـاوبونتو

So, how can I use underline in Persian language?