OK. Just found out that I have to use the idx parameter names in the API call. The correct call is: http://ipadress/co?pw=mypassword&o2=1&o3=1&o32=1&o23=77. The API Document isn’t so easy to understand? However, looking at it after some time again provides the solution for me.
I hope that this is at least an example for others on how to change the water level via the OS API call without any side effects.