Check the System Useragent box first, then click Manage UserAgent button, then you can find many default useragents in this program.
Uncheck the System UserAgent box, then click Manage UserAgent button to add yourself useragent list. You can search some useragent online.
Type whatismyuseragent in your browser, then you can find what the useragent is.
Here is an example:
Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/66.0.3359.139 Safari/537.36
Ok, I found a list here;
https://techblog.willshouse.com/2012/01/03/most-common-user-agents/
Thank you.