In the initialization function for the shared library that defines the new tag, register the tag using the function shtml_add_tag. The signature is:
NSAPI_PUBLIC int shtml_add_tag ( const char* tag, ShtmlTagInstanceLoad ctor, ShtmlTagInstanceUnload dtor, ShtmlTagExecuteFunc execFn, ShtmlTagPageLoadFunc pageLoadFn, ShtmlTagPageUnLoadFunc pageUnLoadFn);
Any of these arguments can return NULL except for the tag and execFn.