Open your Zenoti Webstore in a browser, choose a location and open the service. The address bar gains ?serviceid= followed by a long code. Copy everything after serviceid= and paste it into the block's Booking ID field with Link type set to Service.
Category IDs appear after ?CategoryId=, membership IDs after /sales/membership/ and package IDs after /sales/seriespackage/ on their own pages.
If the address bar does not change, open the service in a new tab and the full address appears. Your Zenoti administrator can also export a list of services with their IDs.
The complete guide, with a diagram of the address bar, is Where to find service, category, membership and package IDs.