How to change the text of the "OK" button of the popup that displays the dropdown_search package in Flutter

You can’t modify actual text on that button, but it’s possible to modify button creation.

This is how button created: source code

You can provide similar method as validationBuilder parameter to you popupProps. Like this:

DropdownSearch<String>.multiSelection(
  popupProps: PopupPropsMultiSelection.menu(
    showSearchBox: false,
    showSelectedItems: false,
    fit: FlexFit.loose,
    validationBuilder: (context, items) => Padding(
      padding: EdgeInsets.all(8),
      child: Align(
        alignment: Alignment.centerRight,
        child: ElevatedButton(
          onPressed: () {
            // do something here with items
            // Also you can close dialog here with
            // Navigator.pop(context);
            // default implementation does 2 things:
            // 1. calls onChanged method with items as parameter
            // 2. Closes popup with Navigator.pop(context);
          },
          child: Text("Your text"),
        ),
      ),
    ),
  ),
);

Pay attention to onPressed – you will have to provide actual logic of closing this menu and manipulate chosen items. I left a comment with suggestion and more or less default implementation.