I can't figure out how to write correctly
Either:
open the console and enter:
addnode 213.31.19.87 add
addnode 89.73.143.76 add
addnode 144.76.118.44 add
Or add the lines:
addnode=213.31.19.87
addnode=89.73.143.76
to the
datacoin.conf file - see example datacoin.conf here:
https://github.com/datacoinproject/datacoin/releases/download/0.16.3.1/example.datacoin.confwrites an error in the console
15:13:42

addnode 213.31.19.87 add
15:13:42

Error: Node already added (code -23)
Cheers
Graham