Looking for logging into the System_client site and are not getting the correct working link for it? In such a case, you can use the links given here by searching your desired login page in the search box. If the page is present on our website then you will get all the links related to the System_client site. After that you can login as usual with your login credentials.
https://archive.sap.com/discussions/thread/1062137
https://wiki.scn.sap.com/wiki/display/Basis/How+To+Change+Default+Logon+Client
https://help.sap.com/doc/saphelp_nw73/7.3.16/en-US/4a/c3f18f8c352470e10000000a42189c/content.htm
https://itsiti.com/login-system_client
https://www.consolut.com/en/s/sap-ides-access/d/s/doc/L-LOGIN$SYSTEM_CLIENT/
https://www.youtube.com/watch?v=7GuSIDwQosU
https://it.toolbox.com/question/how-to-change-default-client-032712
https://it.toolbox.com/question/wrong-client-no-is-displayed-on-sap-easy-access-home-screen-013115
https://www.stechno.net/sap-parameters.html?id=LOGIN_SYSTEM_CLIENT
https://www.tcodesearch.com/sap-profile-parameters/detail?id=LOGIN/SYSTEM_CLIENT
http://www.tcodesearch.com/sap-profile-parameters/search?q=recommended+value+for+login/system_client
http://sap.work/tag/login-system_client/
http://www.sapbasisadmin.com/tag/loginsystem_client-parameter/