//Models:
class Een extends Model
{
    use HasFactory;
    protected $fillable = ['name'];

    public function Twees()
    {
        return $this->hasMany(Twee::class);
    }
}
class Twee extends Model
{
    use HasFactory;

    protected $fillable = ['name','een_id'];

    public function Een()
    {
        return $this->belongsTo(Een::class);
    }
}
//TweeResource:
 public static function getFormFields()
    {
        return [
            TextInput::make('name'),
            Select::make('een_id')->relationship(name:'Een', titleAttribute:'name')
        ];
    }

    public static function form(Form $form): Form
    {
        return $form
            ->schema(self::getFormFields());
    }
//EenResource:
    public static function table(Table $table): Table
    {
        return $table
            ->columns([
                TextColumn::make('name')
            ])
            ->actions([
                Tables\Actions\EditAction::make(),
                Tables\Actions\Action::make('foo')
                ->button()
                ->model(TweeResource::getModel())
                ->form(TweeResource::getFormFields())
                ->mountUsing(
                    function (Form $form) {
                        $een = Een::first();
                        $form->fill([
                            'name'     => 'hello',
                            'een_id'   => $een->id,
                            'een.name' => $een->name]);
                    }
                )
            ])