Note that a log file must not be placed in the program folder: a non-administrative user does not have sufficient priviledges to write it. Use e.g.
%appdata%
. That can be expanded by the Environment class.
<add key="LogFilePath" value="%appdata%\YourCompany\Logs\xy.log" />
string val = ConfigurationManager.AppSettings["LogFilePath"];
string path = System.Environment.ExpandEnvironmentVariables(val);