I'm just a user like you, so I may be wrong...
I was just curious, have you actually succeeded in sending just text, using the REST api to your device? Reason for asking is that usually with REST it is the device that initiates CRUD requests and the server merely responds to that request. I'm assuming then you are referring to getting the wia.io 200 response to contain JSON data.
If you want to avoid your device polling wia.io to check for data then suggest using the MQTT pubsub option.
Either way for an LED matrix all you need to send is a 1x64 data array. This array could be generated using Flows (functions) or insert the data in Events using a unique key to link the values.