how to keet get parameters upong form submition

The current url is: http://localhost:8000/?sort=age and I have a filter in the page.

<form method="GET">
Filter By Name:<input name="filter" id="filter" type="text"/>
<input type="submit" value="Filter">
</form>

when the filter is applied, I expect the url to become: http://localhost:8000/?sort=age&filter=maria but it changes the url to: http://localhost:8000/?filter=maria

Is there a way to keep the current parameters?