XMP CustomFileInfo Panel using MetadataUI: challenges on having multiselect dropdown field

I'm sorry to tell you that the dropdown can only have one selection. You will have to use a list box instead. You might not want a bunch of big list boxes filling up your UI, so you could create a pseudo drop down by setting the listbox height to 0 and using a button that expands it to a larger height revealing the list. On a button click, or some other event, the list box could go back to a height of 0 and save the selected values.

Here is a sample solution, but there are a lot of other possibilities. There is some weirdness in ScriptUI controlls like preferredSize, minimumSize, and maximumSize, so you might have to try different combinations to get the desired display in Windows or Mac.

var win = new Window('palette',"listbox as dropdown", undefined, {closeButton:true});

var keywords = ["one", "two", "three","four","five","six","seven"]

list1Grp = win.add('group');
list1Grp.orientation = 'column';
list1Grp.alignChildren = 'left';
list1Grp.spacing = 0;

list2Grp = list1Grp.add('group');
list2Grp.orientation = 'row';
list2Grp.alignChildren = 'left';
list2Grp.spacing = 0;

list1Txt = list2Grp.add('statictext',undefined,"Select Keywords");
list1Txt.preferredSize=[280,25];
list1Btn = list2Grp.add('button',undefined,"🔻");
list1Btn.preferredSize=[20,20];

var list1 =list1Grp.add ('listbox', undefined, keywords, {multiselect: true});
list1.preferredSize=[280,0];

list2Btn = list1Grp.add('button',undefined,"Add Keywords");
list2Btn.preferredSize=[280,20];
list2Btn.visible = false;

list1Btn.onClick=function(){
    list1.minimumSize=[280,150];
    list1.maximumSize=[280,150];
    list2Btn.visible = true;
    win.layout.layout (true);
    }

list2Btn.onClick=function(){
    if(list1.selection == null){
        list1Txt.text = "Select Keywords";
        }
     else{
        list1Txt.text = list1.selection.join("; ") // Display the list selections. You could  use this to write to XMP or put the list selections in an array to write to XMP
        }
    list1.minimumSize=[280,0];
    list1.maximumSize=[280,0];
    list2Btn.visible = false;
    win.layout.layout (true);
    }

win.show();