Multiline Prompt Window

is there a simple way to get an prompt for multiple lines of text and later iterate over them?

No, it's not simple. Splitting the return value on newlines and iterating over them is left as an exercise to the reader:

function mprompt(message, preset) {
    var
        messageLength = message?message.length:0,
        d = new Window(
'dialog { m: StaticText { characters: '+messageLength+'},'+
' i: EditText { active: true, characters: 80,'+
'   properties: { multiline: true } },'+
'b: Group { c: Button { text: "Cancel" }, '+
'  o: Button { text: "OK" } }'+
'}');
    if (message) { d.m.text = message; }
    if (preset) { d.i.text = preset; }
    // We need to define our own event handler to get
    // newline characters
    // into the multiline field when you hit Return
    d.addEventListener("keydown", function(e) {
        if (e.keyName==='Enter') { d.i.textselection+='\n'; }
        });
    // Size the input box 4 lines high, unless we know better
    if (!preset) {
        d.i.preferredSize = [ -1, d.i.preferredSize[1]*4 ];
    }
    // We cannot allow the default event handler to capture Enter
    d.defaultElement = null;
    // We need our own handler to close the window.
    d.b.o.onClick = function() { d.close(1); };
    if (d.show()===1) {
        return d.i.text;
    } else {
        return null;
    }
}

mprompt("Multi-line prompt");

alert ("Line 1\rLine 2\rLine 3\etc.");