Laravel 5.4 how to assign roles to user by form request?

i have a users list & here i put a option to assign role to each users, for that when click 'Add Role' link it will show this route

Route::get('admin/home/role/{user}', 'RoleController@create');

in create function my form code is...

<form method="post" action="{{ url('admin/home/role') }}">
 {{ csrf_field()   }}


  <div class="form-group">
   <select name="role" class="form-control" >
  <option value="1"> Admin </option>
  <option value="2"> Editor</option>
   </select>
  </div>

   <button type="submit" class="btn btn-primary">
    Add Role
   </button>
 </form>

to manage this form my POST route is...

Route::post('admin/home/role', 'RoleController@store');

now how to insert this form request data into role_user table? oh! i have already 3 table, users, roles & role_user.

User model relationship code is...

 public function role()
  {
  return $this->belongsToMany(Role::class, 'role_user');
  } 

Role model relationship code is...

public function user()
{
return $this->belongsToMany(User::class, 'role_user');
}  

my question is how to insert form request data into role_user table? i know one way that is...

public function store(Request $request, User $user)
{

$role = Role::find(1);
$user = User::find(19);

$role->user()->attach($user);


}

it works, but this is not dynamic. How to insert by form request? please help me. I searching about this topic tutorial but not found.