Approvals on Team-Managed project - how to add user to Approval Field

[Nicolas Favrod-Coune]

Hello,

I nedd to add approvers to an approval step on a Team-Managed project.

The customfield Approvals isn't available on Modify Ticket Action Step. 

Therefore, I try to add accountId with the smartvalues : 

{  "update": {  "{{issue.customfield_xxxxx}}": [  {  "add": { "accountId": "{{issue.customfield_yyyyy.accountid}"}  }  ]  } }

The value of customfields in my automation log are: 

{{issue.customfield_yyyyy.accountid} = "6412f2c767102fc717bfe22e"  
{{issue.customfield_xxxxx}} = "approvals" : "ServiceDeskApprovalsBean{id='10397', name='Validation', finalDecision='pending', canAnswerApproval=false, approvers=[]}"

I've tried many thing but I can't achive adding an account in approvers. 

One of the Error I have is :

ISSUE.KEY  (Field with id' customfield_xxxxx' and name 'Approvals' does not support operation 'add' Supported operation(s) are: 'set')

Can you help me find a way ? 

Very best