PythonでFlaskサーバーを作成し、ユーザーに文字列を送り返すエンドポイントを作成する簡単な例です：

```
from flask import Flask, request

app = Flask(__name__)

@app.route('/', methods=['POST'])
def echo():
    request_body = request.get_json(force=True)
    return request_body['input']

app.run(port=3000)
```

このアプリケーションはcurlでテストできます：
```
curl -X POST -H 'Content-Type: application/json' -d '{"input": "Hello server"}' http://127.0.0.1:3000/
```
Here's a simple example of a Flask server in Python which creates an endpoint which sends a string back to a user:

```
from flask import Flask, request

app = Flask(__name__)

@app.route('/', methods=['POST'])
def echo():
    request_body = request.get_json(force=True)
    return request_body['input']

app.run(port=3000)
```

This application can be tested with curl as:
```
curl -X POST -H 'Content-Type: application/json' -d '{"input": "Hello server"}' http://127.0.0.1:3000/
```