Sir, I didn't know this was related to the SA:MP Client. Oh, wait! Thats because it isn't. You cannot do this within SA:MP, and it actually has nothing to do with SA:MP. You need to add a normal DNS record.
If you use godaddy.com, heres a demonstration:
Click "edit zone".
Then: look for an A record section. Hit "Add Record". Something like this will come up then:
Then: accordingly modify the record, and apply the changes.
It should be generally the same with all domain registrars.
NOTE: It can take up to 2 hours for DNS records to properly propagate, how-ever GoDaddy updates pretty fast.