Not documented (not sure why), but implemented: https://github.com/ClickHouse/ClickHouse/issues/8842#issuecomment-1145888337
> curl -X POST -F 'query=select {p1:UInt8} + {p2:UInt8}' -F "param_p1=3" -F "param_p2=4" 'http://default:default@localhost:8123/'