From LedHed's Wiki
-- Overview -- How to create a CSR on macOS. Useful for 802.1x, VPN, and more.
-- Keychain --
- Using 'Finder' open: Keychain Access
- Click 'Keychain Access' in the upper left, and select: 'Certificate Assistant', then click: 'Create a Certificate from a Certificate Authority'
-- Reference --
https://www.ssl.com/how-to/csr-generation-in-macos-keychain-access/