Get data won't insert - php

Question

I've been using $_GET data to send data across pages and it has been working fine.
What i have on one page is news. Each news article has its own specific ID (and this page works perfectly fine). I can click on an add me button next to each event to add myself as a volunteer for organising the BBQ for that event. However now i'm trying to click on an add button which can add other users to the BBQ.
I've checked to see if the $_GET data is returning anything on page load and it does, however the values are lost when i click submit. So, when i check to see if it returns anything inside the isset($_POST['userselect']), the values are lost:
Here is my code:
$rosterID = $_GET["rosterid"];
$eventID = $_GET["eventid"];
//if i check to see if they gets work here, they do.
if (hasRole2($connection, "Admin") || hasRole2($connection, "Moderator") || hasRole2($connection, "BBQ Moderator")){
$usernames[] = array();
if ($stmt = $connection->prepare("SELECT id, uid from people")){
$stmt->execute();
$stmt->bind_result($id, $username);
$stmt->store_result();
$form = new jqmForm();
$form->method('post');
$sel = $form->add(new jqmSelect('userselect','userselect','<p align="center">Select User:</p>'), true);
while ($stmt->fetch()){
$usernames[] = array('uid' => $username, 'id' => $id);
$optName = $username;
$optValue = $id;
$sel->add(new jqmOption($optName, $optValue, false));
$sel->attribute('data-native-menu', 'false');
}
$stmt->close();
$form->add(new jqmInput('submit', 'submit', 'submit', 'submit', '', 'b', true));
}
if (isset($_POST["userselect"])){
//if i check to see if the gets work here, they don't.
$personID = $_POST["userselect"];
if (rostered($connection, $personID, $rosterID, $eventID)){
$personID = $_POST["userselect"];
$p->addContent("<p align=center><font color = red>You have already rostered for this event</font></p>");
$login = $p->addContent("<font color=brown size=4><a href = news.php rel=external> Go back </a></font>");
$login->attribute('align', 'center');
}
else{
$search = "INSERT INTO RosterPeopleEvent (roster_id, person_id, news_id) VALUES (?, ?, ?)";
if (!$roster = $connection->prepare($search)){
$p->addContent("Inserting into RosterPeopleEvent Prepare failed: (" . $connection->errno . ") " . $connection->error);
}
else{
$roster->bind_param("iii", $_GET["rosterid"], $personID, $_GET["eventid"]);
$roster->execute();
}
}
}
}

Answer 1

Just added:
$form->action("url.php?rosterid=$rosterID&eventid=$eventID");
This worked.