Placeholder/Hint in PDF Form


I’ve made a fillable PDF with some textBox and checkBox.
I’d like to see in textBox a small hint (or placeholder ? not sure which term to use) like “Allowed format : 0A00A” or whatever, which disappears when user set the focus on it.
I’ve tried with “Help Text” and “Additional information”, but it doesn’t seem to be that…
If I set a default text, the user will have to erase the text before being able to write, and that’s not what I’m looking for.

Example :

Is it possible ?

Thanks !

(I’m on LibreOffice

No way to do that… :’(

The default text is the closest thing to this.