Sigfox Downlink

Sigfox Downlink

Hello guys,
I'm trying to develop a interface with my Sigfox devices. It's ok for Uplink, with my parcer running in js and dashboard being updated. But I still need to return some bytes to my device (Downlink). 
Using the Configuration Parameters downlinkData and a fixed value, the device received the data, then all the sigfox callback config is working. My doubt is how to link the input form to compouse the 8 bytes sentence for downlink? I saw at website that I should use Analysis, but no other information about. Any example?
I'm looking forward your answer.
Thanks and best regards...
Ivan