You need to set the service of the payload in a function node or change node.
[{"id":"02e2a009cbc6ed5b","type":"server-state-changed","z":"ebaa69a9.649708","name":"","server":"4bbca37b.1700ec","version":3,"exposeToHomeAssistant":false,"haConfig":[{"property":"name","value":""},{"property":"icon","value":""}],"entityidfilter":"light.kokslampa","entityidfiltertype":"exact","outputinitially":false,"state_type":"str","haltifstate":"","halt_if_type":"str","halt_if_compare":"is","outputs":1,"output_only_on_state_change":true,"for":0,"forType":"num","forUnits":"minutes","ignorePrevStateNull":false,"ignorePrevStateUnknown":false,"ignorePrevStateUnavailable":false,"ignoreCurrentStateUnknown":false,"ignoreCurrentStateUnavailable":false,"outputProperties":[{"property":"payload","propertyType":"msg","value":"","valueType":"entityState"},{"property":"data","propertyType":"msg","value":"","valueType":"eventData"},{"property":"topic","propertyType":"msg","value":"","valueType":"triggerId"}],"x":200,"y":660,"wires":[["172ad0e5b1849801"]]},{"id":"172ad0e5b1849801","type":"function","z":"ebaa69a9.649708","name":"","func":"msg.payload = { \"service\": \"turn_\" + msg.payload }\n\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":460,"y":660,"wires":[["1b23f207f8e853d5"]]},{"id":"1b23f207f8e853d5","type":"api-call-service","z":"ebaa69a9.649708","name":"","server":"4bbca37b.1700ec","version":3,"debugenabled":false,"service_domain":"switch","service":"","entityId":"switch.hallen","data":"","dataType":"jsonata","mergecontext":"","mustacheAltTags":false,"outputProperties":[],"queue":"none","x":660,"y":660,"wires":[[]]},{"id":"4bbca37b.1700ec","type":"server","name":"Home Assistant","version":1,"legacy":false,"addon":true,"rejectUnauthorizedCerts":true,"ha_boolean":"y|yes|true|on|home|open","connectionDelay":true,"cacheJson":true}]