You can buy ES tickets 🇫🇷-🇧🇪 and 🇧🇪-🇫🇷 at their website or at SNCB-NMBS. SNCB-NMBS also allows you to buy to/from Liège-Guillemins: www.b-europe.com/EN

Additionally according to ČD site this train also stops at Liège-Guillemins, and SNCB-NMBS even selling tickets:
Paris-Berlin: www.cd.cz/en/vlak/496/...
Berlin-Paris: www.cd.cz/en/vlak/497/...

You can now buy tickets to/from Liège-Guillemins at SNCB-NMBS website: www.b-europe.com/EN

ČD site mentions stopping at Liège-Guillemins:
Paris-Berlin www.cd.cz/en/vlak/496/...
Berlin-Paris www.cd.cz/en/vlak/497/...