Can I use rewritehostport() function with argument selected at time of call processing?

I have a lua script, which gets address/port of next hop(from base) But how to call rewritehostport() with it? I can't find, how to call this function from lua script directly (I've tried sr.tm.rewritehostport() and rewritehostport() - no success) If i set pseudovariable (f.e $avp(relay)) and call rewritehostport("$avp(relay)") from kamailio.cfg later, i can see in logs such message:

ERROR: tm [ut.h:327]: failed to resolve "$avp(relay)"

Seems it use name of pv, not value? What should I do to call this function with correct parameter?