The Disks application can be used to encrypt the extra drive and it comes pre-installed on Pop!_OS and Ubuntu.
In this screenshot the extra drive can be seen and then select the Gear icon under Volumes. Then select the Format Partition… option to open the ‘Format Volume’ window.
Then use the ‘Format Volume’ window to format the volume with the options to name the volume and use the Ext4 partition with LUKS.
If we would like to decrypt the drive on login into the system there are settings for the encryption. In the Disks application on the drive that is encrypted then click on the LUKS partition and then the Gear icons on the bottom of the Volumes table. Then click on Edit Encryption Options… then unclick ‘User Session Defaults’. Now enter the Passphrase for the encrypted drive and hit OK button. It will ask for your password and once it is entered the drive will be decrypted on login.
The Extra Drive article goes over changing the mounting options for the drive.