Works, needs dotjs. Could use polish.

fightcodegame.com.js

var replayURL = 'http://fightcodegame.com/robots/replay/'
var ai = 'duck'

function load(event) {
    event.preventDefault()
    var player = document.getElementById('playerNumber')
    $.get(replayURL+$('#replayID').val()).done(function (data) {
        $('#'+ai+'-code').text(
            $(data).filter('[id*="-code"]').eq(
                player.options[player.selectedIndex].value
            ).text()
        )
    })
}
function insertInputs() {
    if ($('.editor').length) {
        var form = $('<form>')
        form.addClass('form')
        form.append($('<input>').attr({
            id: 'replayID',
        }))
        form.append($('<select>').attr({
            id: 'playerNumber'
        }).append($('<option>').attr({
            value: '0',
            name: '1'
        }).text('1')).append($('<option>').attr({
            value: '1',
            name: '2'
        }).text('2')))
        form.append($('<button>').text('Load Bot').click(load))
        $('.buttons-bar').append(form)
    }
}
insertInputs()