There are two things you need to do to make this work (CSF needs to know what to look for and where to look for it)
1 - You can add as many regex statements as needed to /usr/local/csf/bin/regex.custom.pm and in those statements you specify where you want CSF to look for the pattern (CUSTOM1_LOG, CUSTOM2_LOG...). If you have multiple logs you want CSF to check for this pattern, you need to repeat the regex statement with a second (or third...) log part of the statement.
2 - You need to tell CSF where to look. Again, this could be one log or multiple logs. You don't create a log file, you point CSF to a log file where the webserver will be logging these patterns that your regex statement looks for. You save these CUSTOM1_LOG paths to the CSF configuration file.