For selection, you'll use a combination of global functions. First, use the acedSSGet() function to obtain the selection set. Then, use the acedSSNameX() function to obtain a subentity GS marker for the selected entity.
int acedSSGet(
const char * str,
const void * pt1,
const void * pt2,
const struct resbuf * filter,
ads_name ss);
int acedSSNameX(
struct resbuf ** rbpp,
const ads_name ss,
const long i);