jupyterlab/00-startup.py

from IPython.display import display, Javascript

def show_popup():
    display(Javascript("""
        require(
            ['base/js/dialog'],
            function(dialog) {
                dialog.modal({
                    title: 'Welcome',
                    body: 'Popup place-holder',
                    buttons: {
                        'Ok': {}
                    }
                });
            }
        );
    """))

show_popup()