By a farbrengen once the pesukim for the Rebbe (at the end of shmoneh esreh) came up. The Rebbe wrote that his pesukim are מאור עינים ישמח לב and מה טובו. I think that was mentioned then, but when it came up, Reb Mendel said unequivocally "The possuk of the Rebbe is מגדיל ישועות מלכו ועושה חסד למשיחו לדוד ולזרעו עד עולם in a way that left no possibility of argument. He said it with תוקף and conviction that he affected me, that even though I couldn't stop saying the pesukim that the Rebbe mentions, I also couldn't not say magdil, so since then (which is many years) I've been saying magdil, and then me'or einayim and mah tovu, and then my pesukim (which is the secret of why my shemoneh esreh takes a little longer....).