MQTT and the Retain function

Hey people, I just started using App Inventor, I managed to do what I wanted, because I saw a problem that I don't know how to solve, I have an automation, in which I activate 8 Relays via mqtt, here everything is, the application doesn't work App Inventor For example, the date of the App, when I open it again, the button that was pressed activating the relay, seems like nothing has been done.
I would like to know how to use the Retain function in the MQTT protocol within App Inventor, is that part that I don't know how to do, can someone help me?
From now on, thanks!

Here is a little help: