To submit a specimen for services, USPTO recommends online advertising or printed material showing a direct association between the trademark and the services. It could be a screenshot or printed website version, including the URL and the access date. You can also submit a scanned copy of brochures, leaflets, or any relevant marketing document related to your service.
Additional options depend on the character of your service. It is also possible to submit the invoice for the service with the trademark included. For the services like restaurants/bars, it is sufficient to send a photo or a scanned copy of a menu. A music band can send a photo from the stage with their logo in the background during a live musical performance, provided they are applying for/own a trademark under the class of live musical entertainment.
The USPTO inspects the websites of the services offered. If it identifies misconduct, the specimen will be rejected immediately. Thus, submitting real websites with services that can be ordered or purchased is necessary.
Submitting a specimen is necessary as part of the Statement of Use (when filing with an 'intent to use', in other words when the brand is not commercially active in the United States yet) and Declaration of Use (when proving the usage of a US registered trademark between 5-6th year after registration).