In short you cannot make her do it.
You can tell her whenever you see her with it undone but is this really worth it if all it causes is an argument or you getting annoyed when she won’t listen? And besides, when you are not around you cannot make her. The best way, I believe, to teach her why it’s important to do it up is to try and develop some kind of intrinsic motivation for her to do so.
- leaving her to deal with the negative consequences she will face of not zipping it up ( getting cold and/or wet , her jacket annoying her sometimes when it waves around as she’s running etc)
- pointing these consequences out to her when you see them happening and remind her that zipping it up would help.
You say she can do the zip herself and she just won’t. I would avoid doing it for her if she can do it on her own. Let her learn through her own experience. If it really causes a problem for her she will learn to fix it herself.