Hi, thanks for reaching out.
Yes, the pattern is always as you wrote. Please note that the User Manual says 'http', but it has to be 'https' (we will update the Manual).
In TOPCAT, please make also sure that you don't select a too large Radius (i.e. arcseconds and arcminutes are fine, but if you request many degrees, you might not get a result table in reasonable time).
As to seeing which Simple Cone Searches are available, you can either issue this command from your terminal (if you have the astro-datalab package installed):
datalab services svc_type=scs
Or, in a notebook on the DL notebook server, you can do the same with:
from dl import queryClient as qc
print(qc.services(svc_type='scs'))
In the future we plan to register our services with the VO Registry, so they should be easier to discover.
Hope this is helpful.
Robert