You should use a similar approach as the one used in the Items Selector available in Tridion World http://www.sdltridionworld.com/community/2011_extensions/itemselector.aspx
1) You will need to register your custom URL popup as a View.
try {
var op = window.opener;
if (op && op.Tridion && op.Tridion.Controls && op.Tridion.Controls.Popup) {
op.Tridion.Controls.Popup.registerView(null, window);
}
}
catch (e) {
}
2) Access to the Windows.Opener (Component View) fields.
var args = window.dialogArguments;
if (args) {
// get current field value
var value = "";
var fields = args.getFields();
if (fields && fields.length > 0) {
var values = fields[0].getValues();
if (values && values.length > 0) {
value = values[0]
}
}
}