Get the Query String from Page Configuration 'Advanced' option.

You can configure the site page with above shown string and fetch it using
below code :

ServiceContextThreadLocal.getServiceContext().getRequest().getParameter("srce")

or

ParamUtil.get(ServiceContextThreadLocal.getServiceContext().getRequest(),
"srce", "")

You can also fetch this in velocity template :

#set ($serviceContext =
$portal.getClass().forName("com.liferay.portal.service.ServiceContextThreadLocal").getServiceContext())

#set ($httpServletRequest = $serviceContext.getRequest())

#set($seminarId =$paramUtil.getLong($httpServletRequest, 'srce'))

link