You need a Flask view that will receive POST data and an HTML form that will send it.
from flask import request
@app.route('/addRegion', methods=['POST'])
def addRegion():
...
return (request.form['projectFilePath'])
<form action="{{ url_for('addRegion') }}" method="post">
Project file path: <input type="text" name="projectFilePath"><br>
<input type="submit" value="Submit">
</form>