In my case I just straight up told the CEO what my plans were - I've only been working here for two years and asked him for his help in this regarding and he was more than obliging.
As far as what I told him, it was simply, direct and to the point about my future plans - over the course of my time in this company, I've gone from learning the ropes to managing my own team of people that produce results. While he was somewhat saddened to hear of it, I told him that I was still in the early stages and that I wouldn't be going anytime soon, so I could make sure that when my timeline became more set in stone, I would make sure to train my successor and that if anything, he hasn't lost someone from the India team, but rather gained a new one-man sales office in Canada! That put a smile on his face and he was quite happy.
Heck, a few months after I broached the subject with the CEO who I work with directly, he told me to get my documentation ready for a B1/B2 US visa, because it could help me out in future and also for the company if I'm available from Canada to come down to help them for their sales initiatives. So now I have that too, all as a result of the conversation.
I know there are a lot of people out there who don't have the most pleasant working experiences with their current employers, but I look at it much the same way as you would in the dating world - you should demonstrate value to win minds. Present your case clearly and transparently and nobody but the most petty and spiteful of people who stand in your way. Either way, look at it as a win - they'll either give you your desired paperwork (good) or they'll find a way to kick you out before you quit (also good, because then you don't have to work in such a hostile environment and you'll have the letter you want.). Maintain that the reason you're doing this doesn't mean you're going to up and quit as soon as the ink is dry on your passport and that you have atleast 12 - 18 months more to go while you get the transition plan in place and you'll doo all you can to make sure that your successor is well trained.