open a new plugin

I would probably do it something like this (wholly untested):

function getFirstFreeInsertIndex() {
    var btns = sf.ui.proTools.selectedTrack.invalidate().insertButtons;
    for (var i = 0; i < 10; i++)
        if (btns[i].value.invalidate().value === "unassigned") return i;
    return -1;
}

function addPlugin({ categoryName, pluginName, }) {

    var index = getFirstFreeInsertIndex();
    if (index < 0)
        throw `Couldn't find a free insert slot`;

    var preferredPaths = [
        'DSP plug-in',
        'multichannel DSP plug-in',
        'multi-mono DSP plug-in',

        'Native plug-in',
        'multichannel Native plug-in',
        'multi-mono Native plug-in',

        'multichannel plug-in',
        'multi-mono plug-in',

        'plug-in',
    ];

    const menuSelector = (/** @type {FlatMenuItem[]} */ items) => {
        for(let p0 of preferredPaths) {
            var item = items.find(i => i.path.length === 3 && i.path[0] === p0 && i.path[1] === categoryName && i.path[2].startsWith(pluginName));
            if (item) return item;
        }
        throw `Couldn't find plugin "${categoryName}" -> "${pluginName}"`;
    };

    var button = sf.ui.proTools.selectedTrack.insertButtons[index];
    button.popupMenuSelect({
        menuSelector: menuSelector,
    });
}

addPlugin({
    categoryName: 'EQ',
    pluginName: 'EQ3 7-Band'
});