If this form is inserted within a BIRT text element, you can set the current value of _id_model using VALUE-OF tag:
<form method="POST" action="https://birt.net/frameset" name="reportForm">
<input type="hidden" name="__report" value="report_name.rptdesign" />
<input type="hidden" name="_id_model" value="<VALUE-OF>params["_id_model"]</VALUE-OF>" />
New Operation: <input type="text" name="_operation">
<input type="submit" value="Modify Model" />
</form>
You also have to set the HTML text element as "Dynamic text":