You don’t do this inside the RouterOS config. Do it in your RADIUS server. Just add a default Auth-Type = Accept action to your RADIUS server config that passes a Framed-Pool attribute back.
To enable this feature, you must have some billing system in place. Either you can make your own solution using PHP/MYSQL and freeradius or use any cheap and good commercial software , One example is RADIUS Manager. Once user is on your network, and he try to open any site, he will be redirected to your htospto page, where he will be inform that if he is new user, he should go to SIGN UP page to get him self registered, then using SELF SIGNUP form, that user can register his account, refill his account or so on.