Format
#define URL "byond://byond.com:6000"
Defining a URL in a script, specifies the world to connect to when the script is executed by the player. This is referred to as a connection script, because the player uses it to connect to a world. Other post-connection scripts such as byond.dms or a script loaded through client.script are only used to configure the client after it has connected to a world. In those cases, the URL setting has no effect.
It is important to enclose the URL in double quotes. Otherwise, the // symbol would be mistaken for a comment.