To send your public key by email in Mailfence web-interface:
- Go to your Messages and click on New.
- Click on the drop-down in the From field, to select the sender address, for which you would like to share your OpenPGP public key.
- Go to Attachment (in right section) and click on Attach my public key.
You can additionally always Digitally sign the email message with the respective OpenPGP private key, to assure the recipient that you are indeed the owner of that OpenPGP key pair. Some of the other ways of exchanging OpenPGP public key can be found in this blogpost.