In order to add OpenPGP public key from Public key servers. Go to Settings -> Messages -> Encryption:

1. Click on Add public key.

2. Click on Search in public key servers.

• Write down the email ID of your contact person and click on Search.
  • This will search your contact person OpenPGP public key via WKD method.
  • If the key is not found using WKD method, then the search will be made on keys.openpgp.org server (check their privacy policy).
• Select the right public key and click on Import.
  Note: Keys with no User ID (display name and/or email address) are presently not processable on our side. We plan to address this in future.

Make sure to add the right OpenPGP public key by verifying its fingerprint.

You can also import key using file or text. See also this blogpost.