HTTP output plugin failed to execute

Trying to use the http output plugin and have some basic newbie questions on the payload. I am getting these errors when trying to use the plugin.

Jul 28 13:57:00 nt-logstash logstash[335749]: [2022-07-28T13:57:00,180][ERROR][logstash.agent           ] Failed to execute action {:action=>LogStash::PipelineAction::Create/pipeline_id:itentialCommitQueue, :exception=>"Java::JavaLang::IllegalStateException", :message=>"Unable to configure plugins: (ConfigurationError) Something is wrong with your configuration.", :backtrace=>["org.logstash.config.ir.CompiledPipeline.<init>(CompiledPipeline.java:120)", "org.logstash.execution.JavaBasePipelineExt.initialize(JavaBasePipelineExt.java:85)", "org.logstash.execution.JavaBasePipelineExt$INVOKER$i$1$0$initialize.call(JavaBasePipelineExt$INVOKER$i$1$0$initialize.gen)", "org.jruby.internal.runtime.methods.JavaMethod$JavaMethodN.call(JavaMethod.java:837)", "org.jruby.ir.runtime.IRRuntimeHelpers.instanceSuper(IRRuntimeHelpers.java:1169)", "org.jruby.ir.runtime.IRRuntimeHelpers.instanceSuperSplatArgs(IRRuntimeHelpers.java:1156)", "org.jruby.ir.targets.InstanceSuperInvokeSite.invoke(InstanceSuperInvokeSite.java:39)", "usr.share.logstash.logstash_minus_core.lib.logstash.java_pipeline.RUBY$method$initialize$0(/usr/share/logstash/logstash-core/lib/logstash/java_pipeline.rb:48)", "org.jruby.internal.runtime.methods.CompiledIRMethod.call(CompiledIRMethod.java:80)", "org.jruby.internal.runtime.methods.MixedModeIRMethod.call(MixedModeIRMethod.java:70)", "org.jruby.ir.targets.InvokeSite.invoke(InvokeSite.java:207)", "usr.share.logstash.logstash_minus_core.lib.logstash.pipeline_action.create.RUBY$method$execute$0(/usr/share/logstash/logstash-core/lib/logstash/pipeline_action/create.rb:50)", "usr.share.logstash.logstash_minus_core.lib.logstash.agent.RUBY$block$converge_state$2(/usr/share/logstash/logstash-core/lib/logstash/agent.rb:381)", "org.jruby.runtime.CompiledIRBlockBody.callDirect(CompiledIRBlockBody.java:138)", "org.jruby.runtime.IRBlockBody.call(IRBlockBody.java:58)", "org.jruby.runtime.IRBlockBody.call(IRBlockBody.java:52)", "org.jruby.runtime.Block.call(Block.java:139)", "org.jruby.RubyProc.call(RubyProc.java:318)", "org.jruby.internal.runtime.RubyRunnable.run(RubyRunnable.java:105)", "java.base/java.lang.Thread.run(Thread.java:829)"]}

Config

output {
        rabbitmq {
                exchange => "amq.direct"
                exchange_type => "direct"
                durable => true
                passive => true # had to patch the stock plugin for this to work
                key => 'to-ls-syslog'
                host => ["localhost"]
                user => "logstash"
                password => "logstash"
                heartbeat => 10
        }
        http {
                url => "https://10.10.10.10:3443/workflow_engine/startJobWithOptions/test-logstash"
                http_method => "post"
                format => "json"
                # mapping => [ "variables", "event", "%{message}" ]
                headers => ['Authorization', 'Basic AAAAAAAAAAAAAAAAAAAAAA==']
                verify_ssl => false
        } 
}

The api call is to an external system that looks for a payload similar to this

payload = json.dumps({
  "options": {
    "description": "Example job description",
    "variables": {
      "event": {
        "data": "example data",
        "more_data": "example more data"
      }
    }
  }
})

How can I test logstash to see what the payload is for #{message}? Or format the http plugin to look like the request above with the "event" key?