app/Http/Middleware/isCustomUser.php

<?php

namespace App\Http\Middleware;

use Closure;
use Illuminate\Http\Request;
use Symfony\Component\HttpFoundation\Response;
use Auth;

class isCustomUser
{
    /**
     * Handle an incoming request.
     *
     * @param  \Closure(\Illuminate\Http\Request): (\Symfony\Component\HttpFoundation\Response)  $next
     */
    public function handle(Request $request, Closure $next)
    {
        if(Auth::guard('customuser')->check()){
            return $next($request);
        }

        return redirect(route('login.form'));
    }
}