How do I add fields on Reporting API Custom viewTemplate?

Follow these steps:

1. Create a custom viewTemplate.
2. Patch the custom viewTemplate by calling this API endpoint:
   
   https://openapi.ariba.com/api/procurement-reporting-view/v2/prod/viewTemplates/<nameofCustomViewTemplate>/patch?realm=<RealmName>
   
   On the body part, specify: (note the real field example mixed in the code)
   {
     "selectAttributes": [
       "Attributes1",
       "Attributes2",
       "Attributes3",
       "cus_Attributes1",
       "cus_Attributes2",
       "StartDate",
       "UniqueId"
   ],
     "filterExpressions": [
       {
        "name": "FilterName",
        "field": "filterExpression1",
        "op": ">=",
        "defaultValue": "YYYY-MM-DDTHH:MI:SSZ"
       },
       {
        "name": "createdDateTo",
        "field": "TimeCreated",
        "op": "<=",
        "defaultValue": "2019-05-31T00:00:00Z"
       },
       {
        "name": "StatusInList",
        "field": "StatusString",
        "op": "IN",
        "defaultValue": [
                         "Submitted"
                        ]
       }
    ]
   }
   

1. Only the custom viewTemplate can be modified/patched. The OOTB viewTemplate cannot be modified.
2. Invoke the Metadata API end point to bring back the available Select Fields and Filter Expressions.
3. Notice that on custom view, duplicate fields are not allowed.

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