In one sense, there is nothing you can do to achieve your goal (though you can screw it up). It very much depends on the person who has the tree. If they are a giving, sharing, caring type, you will have a hard time not getting cuttings. If they are not that kind of person, you will have a hard time talking them into giving you cuttings. So, don't beat yourself up to badly, if you do not succeed - you may well not be the problem.
Be a friendly, engaging, normal person, not a whack-job and you will do fine. The rest is up to the owner.
I have found that leaving a note seldom, if ever, works. Better to return when someone is home,.