Hi,
Can you give an example? I think if the url in the flowfile content you can use ExtractText processor where you can create dynamic property and use regex to extract the pattern you need. If the url in an attribute then you can use UpdateAttribute where you can also create dynamic property and use Expression Language function like find(regex) to extract the domain for a given attribute. Example ${FullUrl.find(regex)}. To set the regex for finding domain from url, checkout this link:
https://regex101.com/r/jN6kU2/1