howto: auto create a new flag

Hi,
what is the correct way for me create a new flag through code?

i want a user to be able to click a button, enter a name (for the flag) and have a new flag created so they can make a custom list of nodes with their custom name (and make as many different such lists as they please)

ofcourse, if there is another way to do this i'm interested, but do like flag and use it for some global and pre-defined lists on my site already (so sticking with flag would be good)

thanks,
Joseph