app\main\forms(3).py

#...
from wtforms.validators import DataRequired, Length, Email, Regexp, URL
#...

class ContentForm(FlaskForm):
    collection = SelectField(label='Collection', render_kw={'class': 'input-field'}, coerce=int, validators=[DataRequired()])
    title = StringField(label='Title', validators=[DataRequired()])
    description = StringField(label='Description')
    url = StringField(label='URL', validators=[URL(), DataRequired()])
    submit = SubmitField('Submit')