Yes, the plug in action adds "value1".
Because of this I created another ChatGPT prompt and it seems that this works, because this is not added before the original JSON anymore when I just use execute shell script action, instead of using a plug in action.
It seems that this is the solution:
#!/bin/bash
JSON_PAYLOAD=$(osascript -e 'tell application "Keyboard Maestro Engine" to getvariable "jsoncksequence"')
WEBHOOK_URL="https://hook.eu1.make.com/ffoxs6carni3zqh6b8m8ei2tluf3ymof"
curl -X POST "$WEBHOOK_URL" \
-H "Content-Type: application/json" \
-d "$JSON_PAYLOAD"