import wtforms
import jinja2
from wtforms.fields import TextAreaField


class MyForm(wtforms.Form):
    content = TextAreaField('Text Area')
t = jinja2.Template(
    """{% set f = form.content.process_data("please type content") %}
                    {{ form.content() }}"""
    )
t.render(form=MyForm())
WTForms-How to prepopulate a textarea field?