Trying to create shortcut for a certain plug in to next free insert slot...

Ok. that works! Thanks.

How ever, still trying to figure out how to paste a text via a script or macro in a search field pop up like this:

Would be great in many situations.

Thanks

Alan,
SF is designed to primarily work in the Edit window. The simple workaround would be to create an if/else decision around the code you want to execute: if the mix window is active switch to edit window, execute your script and switch back to the Mix window - Else just execute the script.

Like this:

sf.ui.proTools.appActivateMainWindow();

// First, let's get the selected track name
var originallySelectedTrackNames = sf.ui.proTools.selectedTrackNames;

// This next function  makes sure the selected track is visible in the Edit Window
function ensureFirstSelectedTrackIsVisible() {

    sf.ui.proTools.trackGetByName({
        name: originallySelectedTrackNames[0],
        makeVisible: true
    }).track.trackScrollToView();
}

// Next we wrap your whole script into a single function 

function insertEqInFirstFreeSlot() {
    function getFirstFreeInsertIndex() {
        var btns = sf.ui.proTools.selectedTrack.invalidate().insertButtons;
        for (let i = 0; i < 10; i++)
            if (btns[i].value.invalidate().value === "unassigned") return i;
        throw 0;
    }
    sf.ui.proTools.selectedTrack.trackInsertOrSendSelect({
        insertOrSend: "Insert",
        pluginNumber: getFirstFreeInsertIndex() + 1,
        pluginSelector: items => items.filter(item => item.path[0].indexOf("plug-in") >= 0 && item.path.slice(-1)[0].indexOf("FabFilter Pro-Q 3 ") === 0)[0],
    });
}

// Here we create a new function  that executes everything
function main() {

    //Check if Mix window is visible. If not switch to Edit Window and execute your script, then switch back
    if (sf.ui.proTools.getMenuItem('Window', 'Mix').isMenuChecked) {
        sf.ui.proTools.menuClick({
            menuPath: ["Window", "Edit"],
            targetValue: "Enable",
        });
        ensureFirstSelectedTrackIsVisible();

        insertEqInFirstFreeSlot() // your script

        sf.ui.proTools.menuClick({
            menuPath: ["Window", "Mix"],
            targetValue: "Enable",
        });
    }
    /// Else if the Edit window is the main window, just execute the script
    else {
        ensureFirstSelectedTrackIsVisible();
        insertEqInFirstFreeSlot();
        sf.ui.proTools.trackSelectByName({ names: originallySelectedTrackNames, deselectOthers: true });
    }
}
// Now execute main
main();

Now your script will work in both windows.