[Banksy]

How could I bind activate charge to a keybind ?

[Ranma]

like the voicemenu's activate charge?

bind "kp_home" "voicemenu 1 6"

[Banksy]

yes ! thank you ! I appreciate it. Where do I get all the commands ?

[Biggs]

You can use this as a reference but the numbers will be off by one digit.
http://tf2wiki.net/wiki/Voice_commands

As you'll see in that list activate charge is 7 not 6. That's because it starts counting at 0, not at 1.

First voice menu = voicemenu 0
Second = voicemenu 1
Third = voicemenu 2

The same goes for the second number in the bind as well.
"Medic!" would be "voicemenu 0 0"
"Need a dispenser here" would be "voicemenu 1 4"