app = Flask(__name__)


class TestForm(Form):
    fld1 = HiddenField('Field 1')
fld2 = TextField('Field 2')

@app.route('/', methods=['POST', 'GET'])
def index():
    form = TestForm(request.values, fld1='foo', fld2='bar')
if request.method == 'POST' and form.validate():
    return str(form.data)
return render_template('experiment.html', form=form)
if __name__ == '__main__':
    app.run()
wtforms hidden field value