For example see these 2 I already found out how to do:
#add device
syncthing cli config devices add --device-id 3XEND4W-GNHUMRC-DGTZJ3P-HIF67NS-QULTZDJ-66WJ3ST-OV3VHE4-D6MOLAW
#show id
syncthing device-id
(I have to use the terminal because I cannot access my backup computer in another city and Wayland won’t let me use rustdesk remotely.)