Eldritch Blast
At 1st Level you learn one additional cantrip from the Warlock list or one of the following cantrips: acid splash, chill touch, fire bolt, poison spray, ray of frost, or shocking grasp.
Agonizing Blast
At 2nd Level when you hit a target with your Eldritch Blast cantrip, add your Charisma modifier to each die of the damage it deals.
I wouldn't do it quite this way, because it locks you into your choice of cantrip and leaves you nearly helpless against anything resistant or immune to that type of damage. (There's a reason eldritch blast does force damage.) Instead, I would make it simpler:
Agonizing Blast. At 2nd level, when you deal damage with a warlock cantrip, add your Charisma modifier to each die of damage.
Thus, if you have several attack cantrips, they all benefit and you can switch off between them as you wish. In addition, scale back eldritch blast to 1d8, to put it on par with the other cantrips. The end result would be warlocks losing a bit of DPR, but gaining an invocation slot, which I think is a fair trade.