How do Java devs launch customer-side interfaces via internet/webpage?

I am quite new to coding, and most of what I know is Java. I made a GUI in Eclipse IDE that is meant to act as a cost estimator for contracting repairs, and I use Square Online for my website and I know they have a feature where I can embed HTML code. I have spent a few days now trying to figure out how to get my GUI onto the site, but what it seems like is everywhere I look it's either deprecated software, confusing services w/ little to no documentation, and dead ends. So far, I've gotten it into a .jar file and I can run it from my start panel, but that's about it.

What I want to know is what I have to do to get my calculator onto my website and make it look professional. I don't care if I have to change a few lines and install something, rewrite the code, or even start learning a new language. I just need to know how it's done these days. I don't know if my code is relevant to this (I'll edit it in if anyone asks) but it's basically a swing GUI with some JLabels, JButtons, Jtextfields, and a formula that reads the text field inputs and does a calculation on them when the button is pressed. Thanks so much in advance!