I’ve found the answer myself. I’ll post it here and hope I can help someone with it.
I’ve discovered that OpenHab is actually the modbus master. So I didn’t have to configure anything on the Wago PLC. The Wago PLC is a modbus slave without any configuration.
The Wago 750-881 manual gives the mapping of modbus addresses to PLC addresses. For example %MW0 is at modbus address 12288.
modbus.cfg had to be configured like this:
tcp.wagomwnul.connection=192.168.1.150:502
tcp.wagomwnul.type=holding
tcp.wagomwnul.id=0
tcp.wagomwnul.start=12288
tcp.wagomwnul.length=10
tcp.wago.valuetype=uint16
And read MW0 via:
Number nvrammw0 "My Counter low [%d]" (All) {modbus="wagomwnul:0"}