Substitution for a constant value in CB

It's a substitution. So you need to be sure your auto action has substitutions applied to whatever parameter you're using that in. Ensure substitutions are toggled on for the auto action under the Parameters tab. CB Auto Actions also has a format function to deal with inline translates as described under the Parameters tab. Example as follows.

[cb:parse function="translate"]LANGUAGE_KEY_HERE[/cb:parse]

You should also just be able to use it directly as a language string if you're not using it inline in the middle of content. So for example ensure Translate is set to Yes under Parameters then just set the value of the parameter you're trying to do this in to your language key (e.g. DEV_KEY).

I tried all the different methods but no success so far.
Mind to test if it works on a Request AutoAction with a Get method and the key in the action tab ?
Even the parse function does not function.

It's difficult to debug because the autoaction does not input the result, only errors in the code.

PS: I have added the key only in the default languages/override so that it works for all languages at start

 PS: I have added the key only in the default languages/override so that it works for all languages at start

That's not how language overrides work. They only apply to the language they're added to. If you added it to the default language, but not the French language then when French is loaded the language string will not exist. In multi-lingual setups you need to add it to every language to be used as a constant like this.

ahhhh ok that explains the issue...
Thanks I did not even noticed this point

I can't really think of a way to add a true global constant like that without doing something like adding it to configuration.php maybe and trying to access it via config APIs or maybe in PHP itself just define a constant. We've never really needed to cover such a usecase and is probably unlikely we ever will. The alternative is to make a small plugin that adds a new param to CB configuration. Our footer removal plugin does exactly that so you could use it as a base.