Here is an example of how to pass in the custom property object.

You can find the PropertyNumber by looking at the Spira custom properties page in Template Admin. The PropertyNumber is the number listed in the left most column "Field #".

{
  ...
  "CustomProperties": [
    {
      "PropertyNumber": 1,
      "StringValue": "new string" // used for strings and as the default type
    },
    {
      "PropertyNumber": 2,
      "IntegerValue": 3 // used for integer and number fields, as well as list and user fields
    },
    {
      "PropertyNumber": 3,
      "DateTimeValue": "Thu Oct 31 2019 10:28:11 GMT-0400 (Eastern Daylight Time)" // used for date time fields. Use a standard JSON date object for v6 of the API
    },
    {
      "PropertyNumber": 4,
      "IntegerListValue": [1,2,3] // used for multi select lists
    },
    {
      "PropertyNumber": 4,
      "BooleanValue": false // for boolean fields
    }
  ],
  ...
}