I have spent the most mind numbing hour or two trying to get this to function with any values other than the ones above and it will not have it. If I leave them as they are then it functions perfectly.
The documentation for this isn't great and wondered if anyone had any suggestions or tips?
Following one point in the documentation I changed myendpoint to anything unqiue, I then changed mycustom to mystartof and customendpoint to startofendpoint as it seemed to suggest that the match should be the start of the members value. This also didn't work.
I think I know something that might be causing your headaches. It's something I ran into years ago too with restmap.conf endpoints, and that was unbelievably frustrating and peculiar until I figured it out.
The python class names must be globally unique across all apps and endpoints. If there are two endpoints that have the same python classname, only one of them will be loaded and the other one will be inexplicably doa. Also if during extended troubleshooting sessions and headbanging exercises, you save versions like "myHandler_backup.py", or "myHandler_take3.py", don't. Splunk will try and load those too, I think even if there's no restmap.conf entry associated with it, and when the classnames match another file's classnames, you guessed it, only one will work.