Dropdown menu - user input - select as typing if in menu

Can anyone assist? I currently have a regular exression limiting the users inputed text to just a-z and a char limit:

function sname(){

if (!event.willCommit) {

var value = AFMergeChange(event);

if(!value) return;

if(!AFExactMatch(/^[a-zA-Z \ ]{0,38}/, value)) event.rc = false;
    }
}

But how do I get the combo box to open the predefined list when the field is on focus? for example user tabs to field and on entering the field, the drop down automatically drops down for user to see (instead of click the down arrow on the field).

Any help at all would be greatly appreciated as I need to do this for work.

Thanks

@GKaiseril What if I have keystroke set up that opens the dropdown on focus and as you type it filters the list (if what you type isn't there then it just lets you enter what you are typing)?

I've found this bit of code but it is for livecycle, I need javascript since I am doing this in Acrobat XI Pro.

xfa.host.openList("Sender_Name");

The options listed when the field is created should be available by using the drop down arrow and then if needed the user should be able to add a new item by typing it in the form.

Note the changes to the user's form will carry over to other copies of the form.