Is there an easy way to set a default value for text form field?
You can set the default for related field in your model class (in mapping definition or set the value yourself).
Furthermore, FormBuilder gives you a chance to set initial values with setData() method. Form builder is passed to the createForm() method of your form class.
Also, check this link: http://symfony.com/doc/current/book/forms.html#using-a-form-without-a-class