It’s not possible to load programs via modbus, I’m afraid. You’ll have to set up the PLC to connect to the robot’s dashboard server. I’ve not used a Siemens Logo before, so I don’t know if it’s possible in that PLC.
You can read more here about setting up the connection: Protective Stop - Enable robot from PLC - #18 by v.laurent
Here’s the article on the dashboard server: https://www.universal-robots.com/articles/ur/dashboard-server-e-series-port-29999/