Send Headers in HTTP post request

Elastic Stack Beats
2

Can you try with newline like this?

heartbeat.monitors:
- type: http
# List or urls to query
  urls: ["http://localhost:8724/service/XXX/YYY"]
  # request details:
  check.request.method: POST
  check.request.headers:
    soapaction: '/Processes/XXXX'
  check.request.body: 'XXX'
  # Configure task schedule
  schedule: '@every 10s'

Can you share the debug log from starting heartbeat in foreground with -e -d '*,config'?

Did you check with wireshark the header is missing?