[PLUGIN.PHP]
<?php namespace Alex\Clients;

use System\Classes\PluginBase;
use RainLab\User\Models\User as UserModel;
use Rainlab\User\Controllers\Users as UsersController;
use Alex\Clients\Models\Clients as ClientsModel;

/**
 * Clients Plugin Information File
 */
class Plugin extends PluginBase
{

    /**
     * Returns information about this plugin.
     *
     * @return array
     */
    public function pluginDetails()
    {
        return [
            'name'        => 'Clients',
            'description' => 'No description provided yet...',
            'author'      => 'Alex',
            'icon'        => 'icon-leaf'
        ];
    }

    public function boot()

        {
        UserModel::extend(function($model)
        {
           $model->hasOne['clients'] = ['Alex\Clients\Models\Clients'];
        });

        UsersController::extendFormFields(function($form, $model, $context){

            if(!$model instanceof UserModel)
                return;

            if (!$model->exists)
                return;

            ClientsModel::GetFromUser($model);

            $form->addTabFields([

                'clients[clients]' => [
                    'label' => 'Client',
                    'tab' => 'Client'
                ],

            ]);

        });

    }

}

[CLENTS.PHP]
<?php namespace Alex\Clients\Models;

use Model;

/**
 * Clients Model
 */
class Clients extends Model
{

    /**
     * @var string The database table used by the model.
     */
    public $table = 'alex_clients_clients';

    /**
     * @var array Guarded fields
     */
    protected $guarded = ['*'];

    /**
     * @var array Fillable fields
     */
    protected $fillable = [];

    /**
     * @var array Relations
     */

    public $belongsTo = [
        'user' => ['RainLab\User\Models\User']
    ];

    public static function getFromUser($user)
    {

     if ($user->clients)
         return $user->clients;

        $clients = new static;
        $clients->user = $user;
        $clients->save();

        $user->clients = $clients;

        return $clients;

    }

}

[FIELD.YAML]
# ===================================
#  Form Field Definitions
# ===================================

fields:
    id:
        label: ID
        disabled: true

    clients:
      label: clients

[COLUMNS.YAML]
# ===================================
#  List Column Definitions
# ===================================

columns:
    id:
        label: ID
        searchable: true

    clients:
        label: clients
        searchable: true