Not sure if you mean to use a potty in the cage or to stop her weeing on you.
It is not uncommon for dwarfies to wee on you while handling. They just tend to go when they need and sometimes if they are a bit nervous too it sets them off! They often get better as they become tamer and older and more settled but still have little accidents. Best way to minimise is to make sure they are always up and about if possible before you take them out so they have had a chance to go and to have a towel on your knee which can be easily washed. The wee smells bad as they have very concentrated urine which is normal.
They are not really trainable as such but they often choose to wee in a potty given the chance. Watch where your hamster wees then put a potty full of chinchilla sand in that place and sprinkle some wet stuff in it. Syrians tend to take to it better than dwarfs we find though who still like a wee corner and to piddle all over their wheels then run in a pee shower!!
Hope you manage to persuade her. We put a little pile of carefresh in the wet corner as this is easily scooped out and replaced every second day or so