With this webservice you can retrieve the URL of a sub category. You need to follow those parameters:
user : Pseudo of a forum administrator
pwd : Password of a forum administrator
idforum : ID of the forum where the sub category is located
idcat : ID of the category where the sub category is located
idsubcat : ID of the sub category
Script example based on the sub category retrieval URL webservice
This webservice returns 2 parameters
status : result of the operation. "OK" means that the process has been successful
urlsubcat : URL to retrieve the topic list of the sub category