fbMQTTPublisherEx / fbMQTTSubscriberEx buffer usage

I would like to use the extended-size MQTT function blocks (fbMQTTPublisherEx and fbMQTTSubscriberEx), but I am unable to figure out how to work with the data buffer in a simple and reliable way.

My goal is to convert characters or numeric values into the USINT buffer format required by these function blocks, but all my attempts so far result in compiler errors.
It is unclear how a STRING or REAL value should be properly written into the MQTT_EX_PUBLISHER_BUFFER array.

I would be very grateful if you could provide two small example programs demonstrating:

How to correctly build an outgoing MQTT message using fbMQTTPublisherEx and the USINT buffer.

How to correctly read and process incoming MQTT data using fbMQTTSubscriberEx and its buffer.

Such minimal working examples would greatly help to understand the intended usage of the extended-buffer MQTT function blocks.

Thank you in advance for your assistance.

Kind regards,
Balázs