They will only imprint on you if you are what they see when they first hatch. If you are getting them already hatched, they won't imprint on you.
Is this specific to muscovies? As I have raised hundreds of rescued mallard ducklings and they are very much able to imprint on me, or other humans within the first week even if they have spent a few days with their duck mum who they hatched under. Sometimes even after a week you still get some who imprint on you.
I have my first ever muscovy duckling in my care now (i normally don’t take muscovies but the yellow and black “wild“ duckling a lady called me about when her cat brought it in turned out to be a muscovy when it arrived and not the mallard I was expecting!).
I can’t say for sure that as it has imprinted as I have not put it down on the ground to check if it will follow me etc, but it certainly is very comfortable and happy around me and prefers to be in my company rather than not so seems very similar to how an imprinted Mallard would react to my presence.
The duckling was a few days old when it arrived, old enough to not have an egg tooth any more but not any older than a few days going by it’s size. The lady who bought it to me had seen mother ducks with other ducklings the same color and size on the creek where she lived so I am assuming it had been hatched under its mother and was raised by her for the few days prior to this persons cat grabbing it.
So in my experience ducklings only imprinting on the first, larger than them, creature they see when they hatch - is absolutely a myth.
And within the first week they are usually quite capable of imprinting on other ‘larger than them’ creatures. Some ducklings will imprint on humans more easily than others, but most (mallard derived, anyway) within the first week will definitely imprint on you as long as you spend tine with them every day, holding and cuddling them, and if you house them indoors where they can hear your voice throughout the day from wherever they are housed.