How to use the Module rest_client of openSIPS

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Linux

open-source

Anyone can help me ? When looking at the http://www.opensips.org[^], I want to use the rest_client Module.What bothered me is how to use it.Regard to the example gvien on the website:
"if (!rest_post("http://myserver.org/register_user", "$fU", "text/plain", "$var(body)", "$var(ct)", "$var(rcode)"))";
However when copying this sentence to the script opensips.cfg, getting the error:CRITICAL:core:yyerror: parse error in config file /usr/local/opensips_proxy/etc/opensips/opensips.cfg, line 331, column 45-46: bad arguments for command <rest_post>.
Any real examples for me? Thank you very much.