Updating Subscription Custom Field with invokeurl Not Working

I'm trying to follow the API instructions as per this documentation:

https://www.zoho.com/subscriptions/api/v1/subscription/#update-custom-fields

// Copy reference number to the PBX Domain custom field
params = '{"label":"PBX Domain","value":"string"}';
response = invokeurl
[
url :"https://subscriptions.zoho.com/api/v1/subscriptions/" + subID + "/customfields"
type :POST
parameters: params
connection:"subscriptionsconnection"
content-type:"application/json"
];

However, when I try to execute this, I get back the following (abridged) response:

{
"code":0,
"message":"Custom Fields Updated Successfully.",
"data":[
{
"customfield_id":"240902000017851054",
"value":"",
"max_length":255,
"field_id":"240902000017851054",
"index":1,
"label":"PBX Domain",
}
]
}

As you can see, the value is coming back as empty even though I'm passing the label and a value. What's going on here? Why won't it update with the string value as provided in the json params in the body?

EDIT: I've also tried passing params as follows:

params = '{"custom_fields":[{"value":"string","label":"PBX Domain"}]}';

And result is the same (value is not updated and returns empty). Also, I should mention, that field does already have a value in it and it’s being updated. When using the GET method, I can see the value but with POST it does not update and returns empty.

Thanks!

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