Call external API json body POST

I need to call an external API from a flowable process sending a JSON formatted body.
So I created a process and defined:

requestURL: and filled with the cURL to reach
requestBody: and placed my json
requestHeaders: and added Content-Type: application/json
also, I’ve found somewhere instructions to specify this into the same json ${“name”:“signPDF”,“type”:“json”,“value”: {

Anyway when I call the /runtime/process-instances passing the right processDefinitionId then I get this error:

“message”: “Internal server error”,
“exception”: "Unknown property used in expression: {\n “AccountCredentials”: {\n “oneshot”: 0,\n …

It is like somewhere I should specify that the body is a JSON but where?
Someone could point me into the right direction?

Thank you

Fixed: The process is correct just need to escape $ that is used in some fields. Thanks!