POST - SMS returns error for user

I continually get this error for the required user_id field when trying to POST SMS (https://dialpad.com/api/v2/sms)

Error parsing ProtoRPC request (Unable to parse request content: Message CombinedContainer is missing required field user_id)

I am formatting this call in the body of the request
'{
"user_id": xxxxxxxxxxxx
}'

Any ideas? It's just an single value integer so I wouldn't think it needs special treatment

Oddly, I can get a successful call through Postman so I'm unsure if there's header of some type that I'm missing that Postman is automatically adding

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