How to update custom field values using API

Ooh, the Decimal field type! I just tried this again with a Decimal field instead of Numeric -- If there's no decimal in the number, it looks like it doesn't update the field, I think that might be the last missing piece! 99.99  worked (with the quotes) as a value for me. 

I have tried running this in Microsoft PowerApp Flow, curl, and Postman. All three fail to update the custom field. When I review the output from Flow and Postman the custom field I am attempting to update still has a value of Null rather than the value I am attempting to set.

I did notice that you put quote marks around the value, so I tried that as well. Unfortunately the results did not change. That field is defined as a Decimal field.

Here is the Postman body, which is the same as the PowerApp Flow body.

I think you have some line breaks (\ symbol) missing that indicate a new line in your curl command, but also some missing flags. I'll be totally honest, I am completely self-taught with JSON so this may not be the perfect/prettiest syntax, but it worked for me. I also typically use Postman, and let Postman prettify the JSON for the ticket portion,  but I did test this in this format, on the command line as well, and it worked: 

curl https://{domain}.zendesk.com/api/v2/tickets/{ticket_id}.json \
-H "Content-Type: application/json" \
-d '{
"ticket": {
"custom_fields": [
{
"id": 360048278251,
"value": "9999"
}
]
}
}' \
-v -u {username}/token:{API key} -X PUT

The \ means "Hey this command is not done yet, there's at least one more line", so that curl doesn't just stop reading and fire after getting the first flag and value for it, which I think was the main issue you were hitting.