To begin with please never use words like "it's important" again. If something is important for you doesn't mean that it has to be important for anyone else. And it doesn't mean that all other questions are less important than yours.
Now back to your question. You didn't provided any details about your version of Mibew Messenger, so I suppose that you're talking about our latest (at the moment) release of 2.2.0.
As far as I understand you'll have to create your own custom style based upon the default one, then alter the appropriate template (i.e. chats/<style name>/templates_src/client_side/leave_message/form.handlebars), then build the style, and switch to it.