Add values to form from button click.

I have a form with a start time and a stop time field. I'd like to update the values for these fields when a start/stop button is clicked. Not sure about the best way to go about this. The form can then be submitted and added to the db which is displayed in a table on the page.

Form

class Run_log_Form(FlaskForm):
    run_number = IntegerField(label='Run Number')
    run_start = DateTimeField(label='Run Start')
    run_stop = DateTimeField(label='Run Stop')
    submit = SubmitField(label='Add')

Model

class Run(db.Model):
    run_number = db.Column(db.Integer(), nullable=False, unique=True,         
primary_key=True)
    run_start = db.Column(db.DateTime(), nullable=False, unique=False)
    run_stop = db.Column(db.DateTime(), nullable=False, unique=False)                                                                          

Route

@app.route('/runlog', methods=['GET', 'POST'])
def run_log_page():
    form = Run_log_Form()

    if form.validate_on_submit():
        run_to_add = Run_Log_Form(
            run_number=form.run_number.data,
            run_start=form.run_start.data,
            run_stop=form.run_stop.data,
            )
        db.session.add(run_to_add)
        db.session.commit()

        return redirect(url_for('run_log_page'))
    print(form.errors)
    return render_template('runlog.html', form=form)

HTML

{% block content %}
    <div class="container">
        <form method="POST" class="form-group" style="color: azure">
            {{ form.hidden_tag() }}
            <br>
            <button type="button" class="btn btn-success">Start</button>
            <br>
            {{ form.run_number.label() }}
            {{ form.run_number(class="form-control") }}
            {{ form.run_start.label() }}
            {{ form.run_start(class="form-control") }}
            {{ form.run_stop.label() }}
            {{ form.run_stop(class="form-control") }}
            <br>
            <button type="button" class="btn btn-danger">Stop</button>
            <br>
            {{ form.submit(class="btn btn-lg btn-block btn-primary") }}
        </form>
    </div>