TAP window with Select Service tab visible
The Select Service tab of the TAP load dialogue allows you to choose which TAP service you wish to query. You need to do this before submitting a new TAP query. This tab is essentially the same as the Registry Query Panel of the other VO query windows. Having queried the registry, click on one of the rows to enter its service URL in the TAP URL field at the bottom.
If you know the URL of the TAP service you wish to query, you can enter it directly into the TAP URL field without a registry search.
At time of writing, there are only a couple of dozen TAP services registered, and it is feasible to query the registry for them all, by clicking the Submit Query button without entering any search terms. Note however that for technical reasons (the registry records for some of these services are large) the search can take a while (typically 30sec on a fast link).
Once a service URL has been chosen in one of these ways, you can click the Enter Query button at the bottom (or equivalently the tab header at the top), and you will be moved to the Enter Query tab.