If you don't know the value of the <Initial Agent Registration Password> use these steps to create a new one...
Login to OEM Cloud Control then... Setup - Security - Registration Passwords
Click Create to add a new password. Enter a Description and an Expiry Date.
To secure an Agent using the new password... On the machine where the agent is running...
cd /u01/app/oracle/product/agent13c/agent_inst/bin
./emctl secure agent
If the agent fails to secure then review the logs.../u01/app/oracle/product/agent13c/agent_inst/sysman/log./emctl status agent -secure