Get value from url

Hi Takayuki,
I’mn using this plugin for to get parameters in url.

Ex. https://www.nomesito.it/contact-page/?id_custom=25

So I use [showparam id_custom] for get value.

For insert this value in hide input text i wrote:
[text text-792 id:report_user_username_id class:report_user_username_id "[id_custom]"]
but It not works.

Why?

Thanks.