Create a bookmark to the URL you want to use and replace the arguments with %1 for the first argument, %2 for the second and so on.
For example: http://maps.google.com/maps?f=d&source=s_d&saddr=%1&daddr=%1
Now define a keyword, for example "map". Done.
Now you can get a route from Hyde Park in London to Paris by typing the following: map "Hyde park, London" Paris
Note that spaces separate arguments. To use spaces within an argument, use double-quotes, like in this example.