Log into your Apple developer account here and click on Certificates, Identifiers & Profiles.
On the left side menu under IDENTIFIERS and then click the + button:
Click App IDs > Continue:
Select a type: App and then Continue:
The Registering an App ID screen will appear:
Under Description, specify your app name then select "Explicit" for Bundle ID (as shown above).
For your BUNDLE ID: specify your reverse-domain name string in the format as follows: com.domainname.appname
For instance, if your website was www.mywebsite.com, then your BUNDLE ID would be com.mywebsite.yourappname
THINGS TO NOTE:
If your website domain or app name starts with a number, then append a letter in front. For instance: www.123test.com would be www.a123test.com / for app name '123test' would be 'a123test'
In this scenario, your bundle ID would be com.a123test.a123test
If you have any special characters in your website url such as hyphens ( — ), then remove the hyphen. For instance, if your website says www.my-website.com, then it would be com.mywebsite.yourappname.
Click Continue and Register.
PLEASE NOTE: You may want to copy and save your bundle ID in your notes or a doc file at this time, for easier access in later steps!
Log into MyStudio > Select My Account > Apple > Developer Information > and under STEP 2, paste your Bundle ID (this is case sensitive)
Then let's head back to your Apple Developer Information page and continue setting up your custom app!