How can I get a google IdToken?

I am using this plugin:, but I did everything without firebase.

After the user selects his Gmail correctly I get an object with a lot of different user informations

and on the backend side I use Java and followed this tutorial:

Now I got the problem that I only get an Access Token via the googleplus plugin, but I need an IDToken, but the Tutorial says it is possible to get the IdToken if I send a webclientId. This is the code I used:

.then(res => {
.catch(err => console.error(err));

Maybe there is something wrong with my webclientId? I don’t know.