How do I set a variable to a combo box in UI editor?

I have a script to change the resolution using a combo box, and would like it to show the chosen resolution when the player opens the settings menu after closing it, instead of a default one.

Every time the player changes the resolution then leaves the menu, when they open it again, it shows whatever resolution I set as default on the combo box. It does not change the actual resolution. For example, the player changes the resolution to 1080p, but when they open that menu again, it shows 720p.