{% block content %}
    <div class="row">
        <div class="col-md-4">
            {{ wtf.quick_form(form) }}
        </div>
    </div>
{% endblock %}

from flask_wtf import FlaskForm
from wtforms import StringField, PasswordField, BooleanField, SubmitField, TextAreaField
from wtforms import HiddenField
from wtforms.validators import DataRequired, Length


class MyForm(FlaskForm):
    myhidden = HiddenField()
    textblock = TextAreaField('textblock', validators=[Length(min=0, max=2000)])
    submit = SubmitField('Submit')

form = MyForm()
...
return render_template('form.html', title='my form', form=form)

form.myhidden = "test value"