I want to use Automation Designer to achieve the process mentioned above.
I have completed the steps to open CMD and connect to RMAN, but I don’t know how to configure Automation Designer to execute the following command in RMAN: @C:\RMAN_script\RMANDBHotBackup.rcv
First of all, I don't think you should use Toad to run your daily backups. You should make scripts and put them on the DB server, and schedule them there.
To answer your question though, you should be able to use a "Shell Execute" action to run any program and pass parameters to it.
I have a dropbox folder with some documents I've written about Toad and Oracle stuff. A few of them are about automation designer. There are also some tutorials online about Automation Designer - you can google for them.