JSFiddle - React, Tailwind, and code Playground

HTML

<textarea name='texto1' id="txtBriefing" rows="5" style="font-family: Trebuchet MS; font-size: 16px; width: 394px; height: 100px" ></textarea>
<input name="btnEnvia" id="btnEnvia" type="submit" value="Enviar" style="font-family: Trebuchet MS; font-size: 20px" disabled="disabled" />

JavaScript

$(function(){
   $('#txtBriefing').on('keyup', function(){
      $('#btnEnvia').prop('disabled', ($(this).val().length == 0)); 
   });
});