Since the script seem only to take entire objects as parameter, not selections, you need to make a new object of the selection you wish to run the script on, e.g using the "create" command:

https://pymolwiki.org/index.php/Create

e.g

create newObj, /Obj//A/5-55

yrb newObj

would run yrb on chain A, residues 5 to 55 of Obj

Alternatively, you can run the script on the whole object, then change those parts of the object you do not wish to see in this representation back to a more schematic representation.