Model:

class Placeholder(models.Model):
    name=models.CharField()
    description = models.CharField(blank=True, maxlength=200)
 
    class Admin:
        list_display = ('name', 'description')
        js = (
              '/appmedia/js/jquery.js',
              '/appmedia/js/placeholder.js',
              )

placeholder.js:

$(document).ready(function(){
   $('#id_name').attr("disabled", true);
});