How to update a contact's custom field value via API?

Hello,

I'm, trying to update a contact record using the below PUT statement and BODY and I can successfully update the system fields values but I can't update values for a contact's custom text field:

PUT https://secure.p01.eloqua.com/api/REST/1.0/data/contact/200344

{

  "emailAddress":"gary.aspinall@example.com",

  "id":"300100",

  "businessPhone":"555-999-5555"

  "firstName":"Gary",

  "lastName":"Aspinall",

  "isBounceback":"true",

  "isSubscribed":"true",

  "100453":"Bounce reason goes here",

}

The above body updates the phone number, first name, last name, bounce back status and globally subscribed status but I can't get a custom field which has the ID 100453.

I also tried this but neither option updates in Eloqua: