How to map ID of CaseID from a sucessfull form submit?

Hi, i'm using bonitasoft 7.1.5 and i'm trying to map the Caseid from a successful form Submit to get the id of the next task in the list. In theory it works, when i press submit in the form i can see the caseID that i get from the request, but the variables from wich i get the id remain empty, any idea why?
Example:
* got 2 variables.
- Caseid = null
- InfoCaseID = http://localhost:8080/bonita/API/bpm/humanTask?c=1&p=0&f=caseId={{Caseid.caseId}} (empty)
User submits form.
- Caseid = {"caseId":"30"}
- InfoCaseID = http://localhost:8080/bonita/API/bpm/humanTask?c=1&p=0&f=caseId={{Caseid.caseId}} (remains empty)
any idea on what can i do? Greetings.