Where to add an optional ‘Title’ or ‘Subject’ input?

I have a website, that users send messages to the staff (similar to support tickets). I want the users to be able to add a title or subject to the message.

Most of the users (90%), will not use it. They don't have to do it, and we don't need the title and subject.

But for some users (maybe 10%), it will help them to find the message in the future, by see the title & subject instead of snippet.

I don't want the regular users will think that they have to add a subject. I don't want the users to spend time by thinking about the subject. But I want that some users, who need it, will have the option to add a subject

What's a good way to add this feature?

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