Add foreign key to table after migration has been run in laravel

I have the following migration in my laravel migrations folder that i have already run:

<?php

use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateAdminTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up() {
        Schema::create('Admin' , function($table){
            $table->increments('id');
            $table->mediumText('title');
            $table->text('blog_content');
            $table->char('tag' , 15);
        });
    }
    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down() {
        Schema::drop('Admin');
    }
}

The above migration is for my admin table, what i would really like to do is add a foreign key in my admin table that is associated with my tags table. something like:

 $table->foreign('tag')->references('tag')->on('tags'); 

How do i do this now that i have already run my migration ??

Thank you.

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