No, you must not define the ‘Remote host’.
Because remote Plex clients could theoretically have any possible IP address, depending on where they are located and which ISP they use.
Afterwards, you should open the web app and tell Plex that it shall use this port forwarding, instead of trying to create one automatically, by going to
Settings - Server - Remote Access - ‘Manually specify public port’
fill the checkbox and put 32400 into the input field. Press
‘Retry’
After you defined this port forwarding, you should verify that it’s working by using e.g. canyouseeme.org