set a special character designer

It sounds like you might be doing something easily managed by a variable in unstructured FrameMaker...Are you entering repetitive product names or similar?

In structured FrameMaker you can define prefix and suffix for elements, making this quite easy.

In unstructured/standard FrameMaker, you’re likely stuck with the scripting and other fine workarounds provided.

Finally, I think I would prefer to create these brackets on a text selection by creating a character designer (instead of “find/change”). How to do it in that case ?

It can be scripted. Save this little piece of code as a *.jsx file. Add the script file to FrameMakers Script catalog under “File->Script”. Select a word and run the script.

// Wrap selected text in quotation marks
var doc = app.ActiveDoc;

if (doc.ObjectValid()) {
    var tr = doc.TextSelection;

    // Check if there is actually a selection
    if (tr.beg.offset !== tr.end.offset) {
        
        // We insert at the end first so the start offset doesn't shift
        // Use "\u201D" for a curly closing quote or '"' for a straight one
        doc.AddText(tr.end, "\u201D"); 
        
        // Use "\u201C" for a curly opening quote or '"' for a straight one
        doc.AddText(tr.beg, "\u201C");
        
    } else {
        alert("Please select a word or phrase first.");
    }
} else {
    alert("No active document found.");
}

Now, I would like to select a text and use the find/change function (in “selection” mode) in order to add parenthesis around the selected text. How to do it inside FM :

If the terms (e.g. “apple”) are being defined or first used at some point in the document, further instances could be Xrefs to the defining instance, and it’s easy to apply the quotes and underline as part of the Cross-Reference Format.

This also has the possibly [un]desired benefit of each referring use being a hypertext to the defining instance.

The underline part is easy, but character styles cannot add characters. If you are referring to a trailing or surrounding quotation marks, you should be able to create a unique character style that added the underline, then do a grep find/change at the very end of your formatting to add in the quotation marks.