The basics

example.json

...
"Scripts" : [
    {
        "Path": "example.nut",
        "RunOn": "UI",
        "UICallback": {
            "After": "example"
        }
    }
]

example.nut

void function example()
{
    loebAdd(0, "Example", exampleFunc()) // loebAdd(headerIndex, identifier, functionOnClick)
    // headerIndex can be any number from 0 to 5, corresponding to the headers in the lobby menu: PLAY, CUSTOMIZE, CALLSIGN, NETWORKS, STORE, SETTINGS.
}

This is the minimum required code needed to create a button using loeb. The function example() runs after any UICallback (it won't work otherwise) and adds a button in the PLAY header with the "Example" text that runs the exampleFunc() function on click.