I'd like to have users be able to run a report from a URL but be able to pass a parameter to that report via the URL.
from a URL
And no, a macro won't work.
I would to suggest to test the Rest Api commands, maybe this can fit your purpose.
Check out this splunk answers->
For further information on Splunk Rest Api's check this document -> https://docs.splunk.com/Documentation/Splunk/7.3.2/RESTREF/RESTprolog