2. Click on the Account Settings (Gear icon) in the lower-left corner of the Vault.
3. Navigate to the URL Rules tab and Click Add.
4. Add the sub-domain to the list.
You also have the option of adding the hostname of a server/site or the full URL path to a website for more specific matching. In the case of the full URL path, this could help prevent phishing attacks, because LastPass will only auto-fill on an exact match. Spoofed (faked) URLs will not be an exact match.
Additional Resources:
For more information on using URL rules in LastPass to disable auto-fill for certain sites, refer to the LastPass (LogMeIn) article: Manage Never URLs.