There are two ways how you can find out the Apple Podcast ID.

Via Podcast Connect

Log in at https://podcastsconnect.apple.com/my-podcasts and click on the podcast whose ID you need. The field "Apple Podcasts broadcast ID" will show it to you. 

Via Apple Podcasts on your desktop

Open Apple Podcasts on your computer and go to your podcast (for example via search). On your podcast page you will find the info "Copy link" in the dropdown menu under the "Subscribe" button. The last part of the link (the numbers) is your Apple Podcast ID.