How do I concatenate a String with id and save in Database

You can use the created model event to retrieve the id assigned to the newly created model, then build your manual id and save the model again.

// YourModel.php

public static function boot()
{
    static::created(function ($model) {
        $model->manual_key = 'RGG' . str_pad($model->id, 2,'0',STR_PAD_LEFT);
        $model->save();
    });
}

I've Pasted above code in my model , but what should i write in my controller , i'm getting below error

SQLSTATE[HY000]: General error: 1364 Field 'manual_key' doesn't have a default value

I would probaly use a mutator for that.

migration

            $table->increments('id');
            $table->string('name');
            $table->string('identifier');
            $table->timestamps();

model

public function setIdentifierAttribute($value)
{
    $this->attribute['identifier'] = 'RGG' . str_pad($value, 2,'0',STR_PAD_LEFT);
}