Here is an example of how to pass in the custom property object:
{
...
"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
}
],
...
}