The Open a URL action allows you to open a specified URL with a selected application.
Enter the full URL in the Open URL text area.
Select the application to open the URL from the with pop-up.
Keep in mind that the URL must be properly formatted. It cannot contain spaces or other invalid characters (the Percent Encode for URL filter may be useful for this purpose).
The Gear Menu provides options to control what text processing is applied in the URL block before it is send to the application (7.3+):
Most other Text Fields in Keyboard Maestro process both text tokens and (except for regular expression fields) backslash characters.