diff --git a/src/pyoncatqt/mainwindow.py b/src/pyoncatqt/mainwindow.py
index a25153d..80f3cbf 100644
--- a/src/pyoncatqt/mainwindow.py
+++ b/src/pyoncatqt/mainwindow.py
@@ -10,15 +10,13 @@
 class MainWindow(QWidget):
     """Main widget"""
 
-    def __init__(self: QWidget, parent: QWidget) -> None:
+    def __init__(self: QWidget, key: str = "shiver", parent: QWidget = None) -> None:
         super().__init__(parent=parent)
-        self.initUI()
 
-    def initUI(self: QWidget) -> None:
         layout = QVBoxLayout()
 
         # Create and add the Oncat widget
-        self.oncat_widget = ONCatLogin(key="shiver", parent=self)
+        self.oncat_widget = ONCatLogin(key=key, parent=self)
         self.oncat_widget.connection_updated.connect(self.update_instrument_lists)
         layout.addWidget(self.oncat_widget)
 
diff --git a/src/pyoncatqt/pyoncatqt.py b/src/pyoncatqt/pyoncatqt.py
index 4c3479c..43aa92c 100644
--- a/src/pyoncatqt/pyoncatqt.py
+++ b/src/pyoncatqt/pyoncatqt.py
@@ -3,6 +3,7 @@
 """
 
 import sys
+from typing import Any, Dict
 
 from qtpy.QtWidgets import QApplication, QMainWindow
 
@@ -15,16 +16,16 @@ class PyONCatQt(QMainWindow):
 
     __instance = None
 
-    def __new__(cls: QMainWindow) -> QMainWindow:
+    def __new__(cls: QMainWindow, **kwargs: Dict[str, Any]) -> QMainWindow:  # noqa ARG003
         if PyONCatQt.__instance is None:
             PyONCatQt.__instance = QMainWindow.__new__(cls)
         return PyONCatQt.__instance
 
-    def __init__(self: QMainWindow, parent: QApplication = None) -> None:
+    def __init__(self: QMainWindow, parent: QApplication = None, **kwargs: Dict[str, Any]) -> None:
         super().__init__(parent)
-
+        key = kwargs.pop("key", "shiver")
         self.setWindowTitle(f"PYONCATQT - {__version__}")
-        self.main_window = MainWindow(self)
+        self.main_window = MainWindow(key, parent=self)
         self.setCentralWidget(self.main_window)
 
 
diff --git a/tests/test_main_window.py b/tests/test_main_window.py
index e304120..4bdbb22 100644
--- a/tests/test_main_window.py
+++ b/tests/test_main_window.py
@@ -6,7 +6,7 @@
 
 
 def test_update_instrument_lists(qtbot: pytest.fixture) -> None:
-    main_window = MainWindow(None)
+    main_window = MainWindow("test", None)
     qtbot.addWidget(main_window)
 
     oncat_widget_mock = MagicMock()
diff --git a/tests/test_pyoncatqt_gui.py b/tests/test_pyoncatqt_gui.py
index 3f5ab16..3af232f 100644
--- a/tests/test_pyoncatqt_gui.py
+++ b/tests/test_pyoncatqt_gui.py
@@ -36,7 +36,8 @@ def test_gui(mock_pyoncatqt: mock, mock_qtapp: mock) -> None:
 
 
 def test_PyONCatQt_initialization(qtbot: pytest.fixture) -> None:
-    pyoncatqt = PyONCatQt()
+    kwargs = {"key": "test"}
+    pyoncatqt = PyONCatQt(**kwargs)
     qtbot.addWidget(pyoncatqt)
 
     assert pyoncatqt.windowTitle() == f"PYONCATQT - {__version__}"