add-to-home.js

var deferredPromptEvent; 

window.addEventListener('beforeinstallprompt', function(e) {
    e.preventDefault();
    deferredPromptEvent = e;
    return false;
});

// and in the moment your condition is fulfilled
// check if the prompt had been triggered
if(deferredPromptEvent !== undefined && deferredPromptEvent) {
    // show message
    deferredPromptEvent.prompt();
    // do something on the user choice
    deferredPromptEvent.userChoice.then(function(choiceResult) {
        if(choiceResult.outcome != 'dismissed') {
        }
        // finally remove it
        deferredPromptEvent = null;
    });
}