public class TestPlugin extends JavaPlugin implements Listener {
    @Override
    public void onEnable() {
        getServer().getPluginManager().registerEvents(this, this);
    }

    @EventHandler
    public void on(PlayerInteractAtEntityEvent event) {
        event.getRightClicked().addPassenger(event.getPlayer());
    }

    @EventHandler
    public void on(EntityDismountEvent event) {
        event.setCancelled(true);
        setVehicleBack(event.getEntity(), event.getDismounted());
    }

    private void setVehicleBack(Entity rider, Entity vehicle) {
        try {
            Field ax = net.minecraft.server.v1_13_R1.Entity.class.getDeclaredField("ax");
            ax.setAccessible(true);
            ax.set(((CraftEntity) rider).getHandle(), ((CraftEntity) vehicle).getHandle());
        } catch (NoSuchFieldException | IllegalAccessException e) {
            e.printStackTrace();
        }
    }
}