You should be aware of the fact that your code is vulnerable to SQL Injection, so it must not be used in 'real world' environments but for testing only!
To prevent SQL Injections from the beginning you can use Prepared Statements as described in the docs:
I am not familar with the mysqli driver and would recommend to use PDO, which is a more generic approach for database connection.
However, your code with mysqli prepared statements should look like this:
$dbc = mysqli_connect('localhost', 'username', 'password', 'test_db')
or die("There was an error connecting to the database. Please try again later.");
$fullname = (string)$_POST['name'];
$guest_email = $_POST['email'];
$password = $_POST['password'];
echo "<h1>Thanks for your submission!</h1>";
echo "Your Name on File is: ". $fullname . '<br>';
echo "Your Email on File is: ". $guest_email . '<br>';
//echo "Your Password on File is: ". $password . '<br>'; // never ever print any password!
/* create prepared statement */
$stmt = mysqli_prepare($dbc, "INSERT INTO test_form (full_name, email, user_pass) VALUES(?, ?, ?)");
/* bind params to prepared statement */
mysqli_stmt_bind_param($stmt, 'sss', $fullname, $guest_email, $password);
/* execute prepared statement */
/* close statement and connection */
/* close connection */