What if we do not have the JAR you're looking for, using custom.jar gives you flexibility with just that!
First, we must obtain the JAR from a website! Navigate to your favorite Minecraft JAR website and download the minecraft_startup_server.jar. We'll use paper_1.21.4.jar for this.
Go to https://papermc.io/downloads/paper and download the version of the JAR you would like onto your computer.
Secondly, rename the following JAR to "custom.jar". Exactly like that, and upload onto your PloxHost Minecraft Server via the Panel or FileZilla like application!
Thirdly, we need to modify the panel settings to communicate with our JAR! Go to the Modpacks & Jars => Set Custom Jar and input the custom.jar setting in the blank, and press enter on your keyboard.
Verify that everything is looking as it should and start the server!