add foreign key with existing table in laravel migration code example

Example 1: foreign key laravel migration

$table->foreign('column_name')->references('id')->on('table_name')->onDelete('cascade');

Example 2: laravel migration foreign key 5.6

Schema::table('posts', function (Blueprint $table) {
    $table->unsignedInteger('user_id');

    $table->foreign('user_id')->references('id')->on('users');
});