Keep url-parameter after clicking on submit button in jsp/jstl

I´m working on a jsp page using jstl. In my code, theres a drop down list followed by a checkbox. After hitting the submit button on the drop down list, the url is as following:

/jspsite.jsp?dropdownvariable=dropdownvalue

So far so good. But after hitting the submit button of the following checkbox, the url is as following:

/jspsite.jsp?checkboxvariable=checkboxvalue

But I need to keep both parameters in the url, like this:

/jspsite.jsp?dropdownvariable=dropdownvalue&checkboxvariable=checkboxvalue

So how do I keep both url parameters in my url after hitting both submit buttons?

Is there a way to set a parameter so that it always stays in the url?

Thanks in advance.