To crawl a subdomain of a domain you can create a new project and you can directly enter the path of the subdomain e.g. subdomain.domain.com during the project set up the process.
Before starting a new crawl click on "Crawler-Settings" in the upper right corner. Next, click on "What to crawl". Choose the option "Crawl Subdomains" from the list and make sure that subdomain crawling is deactivated to crawl just the subdomain which you had created before.