Insert on Join Tables Mysql - php

Question

I've seen several questions and answers about this, but none is quite similar to my problem, so now I am asking.
Here is the situation
I have a table called users, roles, and user_role
table: users
fields: user_id, username, password
table: roles
fields: role_id, role_name
table: user_role
fields: user_id, role_id
foreign key of user_role.user_id is user.user_id
foreign key of user_role.role_id is role.role_id
My problem is when I create a user, I also want to set their roles to a default value or a selected value I chose in <select> element.
I tried several things like:
INSERT INTO users as t1 (t1.username, t1.password, t2.role)
JOIN user_role as t2 ON t1.user_id = t2.user_id
VALUES (:username, :password, :role)
Am I doing it super wrong?
EDIT BECAUSE OF AN ANSWER BELOW
I think something is still wrong, it gives me a blank screen
$this->db->beginTransaction();
$string = "INSERT INTO users (username, password)
VALUES (:user,:pass,:first,:middle,:last,:course)";
$sth = $this->db->prepare($string);
$sth->execute(array(
':user'=> $data['user'],
':pass'=> $data['pass']
));
$sth = $this->db->prepare("INSERT INTO user_roles (user_id, role_id)
VALUES (' . $this->db->lastInsertId(user_id) . ', :role)");
$sth->execute(array(
':role' => 3
));
$this->db->commit();

Answer 1

Since it seems like you're using PHP & PDO, this simplified code will take care of your insert:
$pdo->beginTransaction();
$stmt = $conn->prepare('INSERT INTO users (username, password) VALUES (?, ?)');
$stmt->execute(array($username, $password));
$stmt = $conn->prepare('INSERT INTO user_roles (user_id, role_id) VALUES (' . $pdo->lastInsertId() . ', ?)');
$stmt->execute(array($role));
$pdo->commit();
After you insert a user into the users table, the value returned by $pdo->lastInsertId() is the id that was just inserted into the users table.
Read more on lastInsertId().
Encase these two insert statements in a transaction and you're good to go.