Hello Mark,
You had already solved the issue of passing in the values, they are added to the end of the GET call, so you would add ?filter=Today to get a list of the tasks that are for today.
The any supported filter you have highlighted gives examples of the filters that you can apply. I have played around with this and it looks like you can combine filters
tasks?filter=overdue&filter=%23Work
Remember that you will need to url encode spaces as %20 and the # hash is encoded as %23. This should enable you to get back a filtered list using several filters.
As for passing these parameters in Shortcuts, they are just a string that you can add to the end of the url that you are calling.
Or do you mean how to pass a parameter when calling your shortcut from another?
Regards Alan