Intercept submit event with modal window, keep user in page

I have a form that sends data to a google sheet:

<form id="form1" name="form" method="post" 
action="https://script.google.com/macros/s/blablabla/exec">
<div class="forma2">
<input type="email" required name="Email" id="email" placeholder="Email" 
class="forma3 email3">
<input type="submit" name="submiti" value="Send" class="margin-r5 btn2 forma">
</div>
</form>

The below code intercepts the event, gives an alert, and keeps the user to the page:

window.addEventListener("load", function() {
const form = document.getElementById('form1');
form.addEventListener("submit", function(e) {
e.preventDefault();
const data = new FormData(form);
const action = e.target.action;
fetch(action, {
method: 'POST',
body: data,
})
.then(() => {
alert("Success!");
})
});
});

How can I:

(1) display a simple modal window, instead of the above console alert, and

(2) keep the user in the same page?