The link on top of the CU log which reads "Switch to CheckUser main form" should have the user preloaded in the URL
For instance, if you are viewing //wiki/w/index.php?title=Special:CheckUserLog&cuSearchType=target&cuSearch=1.2.3.4 that link should be to //wiki/w/index.php?title=Special:CheckUser&user=1.2.3.4 (currently it is just to //wiki/w/index.php?title=Special:CheckUser)
This allows running a check on that user without having to do a copy-and-paste
The opposite is also true for links from the CU page to the CU log page.