i hope you have followed the previous guide to setup basic router on mikrotik and you have configured the router with dynamic IP address.
IP -> hotspot -> Add servers
IP -> hotspot -> Server Profiles tab
DNS Name: hotspot.zain.pk
IP -> hotspot -> Users tab -> Add New User
IP -> hotspot -> Walled Garden IP List tab
add walled garden ip entry
Dest. Host: cp.zain.pk