Yes we know that. (Although there are stories where people asked for promises from various contemporary Gedolim / Rebbes and were told if you want a promise, not just a Brocha, you can only find that by lubavitch. Don’t know if this is helpful because I don’t know by heart where to find it amongst the thousands of others, I just remember the story itself. But as it’s relevant I thought to mention.)
Let me guess where you can find this story(ies)
In a Lubavitcher Sefer/book