Setting up a Dynamic DNS synthetic record

1. Click My domains in the navbar and then click your domain name.
2. Click the DNS tab .
3. Scroll down to Synthetic Records.
4. Select Dynamic DNS from the list of synthetic record types.
   
5. Enter the name of the resource you plan to have assigned a Dynamic IP, either a subdomain or @ for your default domain (“root domain” or “naked domain”).
   
6. If you selected sub-domain, enter the name of the subdomain.
7. Click Add.
8. The Dynamic DNS record is created in your list of synthetic records. Click the expand triangle to the left of the record to view its values.
   
9. Click View Credentials to view the user name and password created for this record.
   
10. Note the username and password created for the synthetic record. You will need these to configure your gateway or client software to contact the Google name servers.
    
11. You can edit or delete the record with the Edit and Delete buttons next to the record.

Tip: Directing a wild card (*) to a dynamic resource: 

While you cannot use a wildcard in a Dynamic DNS record, you can direct a wildcard to your naked domain or a particular subdomain with a CNAME record.

1. Create a Dynamic DNS record for your naked domain using @ (@.example.com) or for the subdomain you want your wildcard redirected to.
2. Create a CNAME record redirecting * (the wildcard) to your naked domain or subdomain. See Resource records for help with creating a CNAME record.

Source from: https://support.google.com/domains/answer/6147083