I have successfully added field to the process using Rest API's.
when I call the following
with following Json
{ "defaultValue": null, "referenceName": "my.test", "name": "test", "type": "string", "readOnly": false, "required": false, "pickList": null, "url": null, "allowGroups": null }
it returns me the following success msg
{ "referenceName": "my.test", "name": "test", "type": "string", "pickList": null, "readOnly": false, "required": false, "defaultValue": null, "url": "https://my.visualstudio.com/_apis/work/processDefinitions/a949f163-8d2c-4183-b26b-8f5b5385ef15/workItemTypes/my.ProductBacklogItem/fields/my.test", "allowGroups": null }
however in the VSTS the field is not displayed on the workItem