How to customize html side for selectfield(wtforms) flask?

Question

HTML

Good evening to everyone who reads this post.
I want to add a select box to my web site on a flask, but I can't figure out how to set up html-ku for this business.
I'm waiting for any comments and suggestions :)

Main code

class selectmenu(Form):
    month = SelectField('Choose month',choices=[('dec', 'dec'), ('yan', 'yan'), ('feb', 'febt')])
 
@app.route('/searchemp/', methods=['GET', 'POST'])
def searchemp():
    form = selectmenu(request.form)
    m = form.month.data

<form action="" class="form-signin" method="post">
                    <h2 class="form-signin-heading" align="center">title</h2>
                    <input type="text" class="form-control"
                       placeholder= "username" name="username" value="{{request.form.username}}" required autofocus>
                                <!--
                                <input type="text" class="form-control"
                                               placeholder= "month" name="month" value="{{request.form.month}}">
                                -->
                                                         <select name="month">
                                                         <option value="{{request.form.month}}">dec</option>
                                                         <option value="{{request.form.month}}">yanuary</option>
                                                         <option value="{{request.form.month}}">feb</option>
                                                         <option value="{{request.form.month}}">mar</option>
                                                       </select>
 
                    <button class="btn btn-lg btn-success btn-block" type="submit">Search</button>
                    <br>
                    <p align="center">{{error}} </p>
                </form>