This can be done, but it's complicated. There is no simple solution. You can only set a single TFTP server address.
You could set the TFTP server address as the public IP for all TFTP transfers. This might resolve the issue for this one device, but it could prevent other devices from sending configs.
Alternatively you could try to create a customized device command template: Using this other method, you could substituent the variable in the .ConfigMgmt-Commands file ${ServerAddress} with the actual specified public IP address of your device. You need to be careful using this method though, as if your address ever changes, it would break this transfer method. Or if another device of yours tries to use this template (possibly via the AutoDetermine selection), it could fail to transfer the config correctly. I wouldn't recommend this method if you want to backup configs from lots of external devices, as it will just over complicate the environment.
Also you need to remember to open the external port UDP 69 to allow the TFTP server to listen for external requests.