Try to make rbv_api.getHTTPParameter work

Hi,
I got a custom button who link me to a portal page where i pass a parameter (https:// ... /portal.jsp?c=xxxxxxx&p=xxxxxxx&g=xxxxxxx&id=xxxxxxx&cutsomParam=foo)
In a formula field (return text) in Settings, I generate an html code from a request on an OBJECT. I need the paramater to filter this request but i'm not able to make this work.
In this field, at the start I try to get the argument with the following code :

var myParam = rbv_api.getHTTPParameter("cutsomParam");

but myParam always return null. Is this the right way to use the getHTTPParameter function ?

Thanks,
Germain