Visit your site, log in and then edit your URL to look like this:

https://yourdomain.sharepoint.com/sites/yoursite/yoursubsite/_api/web

It the above URL, the following needs to be replaced:
– replace yourdomain with your domain.
– replace yoursite with your sitename.
– replace yoursite with your subsitename.

For example, if your domain is orecons and your site name is test and subsite is test_subsite then URL will look like:

https://orecons.sharepoint.com/sites/test/test_subsite/_api/web

Scroll down to the content section and look for the ID element.

Visit your site, log in and then edit your URL to look like this:

The ID of the site should be under d:Id XML element.