class MyForm(FlaskForm):
...
my_chbox = BooleanField(label=('checkbox '), default=False)
my_clct = SelectField(choices=[...])
...let chbox;
let clct;
chbox = document.getElementById('my_chbox');
clct = document.getElementById('my_clct');
chbox.addEventListener('change', function() {
if (this.checked) {
clct.disabled = false
} else {
clct.disabled = true
}
});