You don't have to use Flask
for this. So if you just want to perform a simple Post Request to a given URL just use the Pythons Request Library like so:
import requests
url = 'someurl'
myobj = {'somekey': 'somevalue'} # if you want to send JSON Data
x = requests.post(url, data = myobj)
print(x.text)
EDIT:
As @OneCricketeer mentioned in his comment you could do this also with the urllib
Package which is included in the Python 3.
from urllib import request
from urllib.parse import urlencode
req = request.Request('your-url', method="POST")
req.add_header('Content-Type', 'application/json') # you could set any header you want with that method
data = urlencode({"key":"value"})
data = data.encode()
response = request.urlopen(req, data=data)
content = response.read()