See https://www.lexaloffle.com/bbs/?pid=75111#p

var customMapFormat = {
    name: "PICO-8 Map Format",
    extension: "p8",
    write: function(map, fileName) {
        var layer = map.layerAt(0);
         var m = 'pico-8 cartridge // http://www.pico-8.com\n'
            + 'version 21\n'
            + '__lua__\n'
            + '-- Exported from Tiled\n\n'
            + '__gfx__\n\n'
            + '__map__\n';
        for (y = 0; y < layer.height; ++y) {
            for (x = 0; x < layer.width; ++x) {
                m += (layer.cellAt(x, y).tileId > 0)
                    ? layer.cellAt(x, y).tileId.toString(16).padStart(2, 0)
                    : '00';
            }
            m += '\n'
        }
        var file = new TextFile(fileName, TextFile.WriteOnly);
        file.write(m);
        file.commit();
    },
}

tiled.registerMapFormat("pico8", customMapFormat)