Is the URL in the regular window or in some kind of list? If it is in the window you can do
Cntrl + L
will take your cursor into the URL window or whatever they call it. and then
Command + left arrow
will take you to the beginning. So you can just automate those key strokes into your macro. Using the "Simulate Keystroke" action. It is diffcult often to place a cursor. If you can highlight something though Cmmnd + left arrow works. I think I might not fully understand your problem. I don't use any RSS feeds now.
There might be a regex way of picking out and transforming URL's in a list type arrangement that is preferable to using the cursor. I don't know the apps and sites you are using. Perhaps you can modify this to take you to the beginning. If you don't know regex though it can be intimidating. I don't know it well enough to be sure I can make one that will work for you without mistakes in it or not cause you problems. This might help though
https://wiki.keyboardmaestro.com/Regular_Expressions?s[]=regex
Getting the cursor onto a very precise particular positon is very difficult to do on Keyboard Maestro I have found. I had a macro that involved getting it into a large side window, that worked ok since there was a broad target. Usually I do it indirectly as it were, by simulating keystrokes. I have a spellcheck macro that works quite well other than I can't get the cursor back to where it left off, long story ask @JMichaelTX he souped the macro up for me and then some!