Heyo,
Firstly, instead of doing this, perhaps you could just make the script heal up with any items in the inventory with an eat option? That way, you wouldn't even need this extra setup step!
If you still want to do it, I would suggest creating your own frame. JOptionPane is fine for small things where you need one or two settings to be saved, but when you start working with more complicated configurations, having multiple JOptionPanes is a bit messy. (You could perhaps extend panel and attach this to your option pane, but I personally prefer creating a whole new frame). You could do this by creating a new class which extends JFrame, and perhaps create and position text fields within this frame in the constructor of the class. There will be plenty of swing tutorials online as it's a pretty standard thing, and you can even get window builders to help you generate the code (often as plugins to IDEs). I would recommend using a layout other than absolute though, since 4k and other higher resolution screens are becoming more widely used so re sizeable windows are very preferable (swing issues :/).
You cannot do this directly to the JOptionPane!
Edit: it looks like you've already created your own frame, do as Chris said! (apologies, I misread your post)
Edit2: You can also (I believe) get rid of that potentially infinite sleep after the mobName initialisation, since I think JOptionPane does that for you. Just be sure to run some string analysis on that variable to make sure it's valid.