Mobile : what is the command to down the keyboard

I need to disable the keyboard to press the button behind

You can use Activities - Execute Command

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Command: mobile:hideKeyboard
Parameters: new Dictionary<string, object>() {} C# syntax

In order to get a list of all the supported functions just execute this activity with a random mobile:unexistentfunction and you should get back a list with all the supported functions. To find out more about these you can go to Command Reference - Appium XCUITest Driver

Note that these commands are driver specific!