Hi manigandan,
The domain in your example looks fine. However, the example does look like there might be other issues. Below I've broken the URL into pieces; the "contact email" key needs to be one word (ie, 'contactemail') and the email address will need to not have a space either.
Code:
https://example.com:2087/xml-api/createacct?
username=myuser&
plan=basic%20&
ip=n&
cpmod=x3&
password=h@rd2gu3ss!p@ss&
contact%20email=username%20@domain.tld& <-- "contactemail=username@domain.tld"
domain=domain.tld&
useregns=0&
reseller=0
Other than that, nothing stands out that could be causing this.
Ensure that all you parameters are correct. If one parameter is off, it could throw the rest off too.
Regards,
-DavidN