You can use the action() helper to generate an URL to your route:
<form method="post" action="{{ action('WelcomeController@log_in') }}" accept-charset="UTF-8">
Note that the Laravel 5 default installation already comes with views and controllers for the whole authentication process. Just go to /home on a fresh install and you should get redirected to a login page.
Also make sure to read the Authentication section in the docs
The error you’re getting now (TokenMismatchException) is because Laravel has CSRF protection out of the box
To make use of it (and make the error go away) add a hidden field to your form:
<input name="_token" type="hidden" value="{{ csrf_token() }}"/>
Alternatively you can also disable CSRF protection by removing 'App\Http\Middleware\VerifyCsrfToken' from the $middleware array in app/Http/Kernel.php