Take a look at the @MULTI@ macro
SlickRun also respects system environment variables, so another option would be to create a environment variable that points to your browser path:
Start > Settings > System > About > Advanced system settings > Environment Variables
Then just use the environment variable in the filename path in SlickRun. Assuming the environment variable you created was called "BrowserPath" then your magic word would look something like: %BrowserPath%\browser.exe (where browser.exe is changed to the actual browser executable.) Then if the path to your browser ever changes you just have to update the environment variable.
The @MULTI@ method is easier to setup, but in case you wanted the Test button in SlickRun to work (it doesn't with @MULTI@ magic words) the Environment Variable method is an option.
Lastly, I'm guessing you're trying to open these URLs in a browser that's not your default? Because otherwise I would think it would make more sense to simply put the URL directly into the "Filename or URL" field for the MagicWord...