The reason you're getting JSON is because you're explicitly calling json.dumps to generate a JSON string. Just don't do that, and you won't get a JSON string. In other words, change your first line to this:

data = {'param1': 'value1', 'param2': 'value2'}

As the docs explain, if you pass a dict as the data value, it will be form-encoded, while if you pass a string, it will be sent as-is.

For example, in one terminal window:

$ nc -kl 8765

In another:

$ python3
>>> import requests
>>> d = {'spam': 20, 'eggs': 3}
>>> requests.post("http://localhost:8765", data=d)
^C
>>> import json
>>> j = json.dumps(d)
>>> requests.post("http://localhost:8765", data=j)
^C

In the first terminal, you'll see that the first request body is this (and Content-Type application/x-www-form-urlencoded):

spam=20&eggs=3

… while the second is this (and has no Content-Type):

{"spam": 20, "eggs": 3}

You can use the Session object

import requests
headers = {'User-Agent': 'Mozilla/5.0'}
payload = {'username':'niceusername','password':'123456'}

session = requests.Session()
session.post('https://admin.example.com/login.php',headers=headers,data=payload)
# the session instance holds the cookie. So use it to get/post later.
# e.g. session.get('https://example.com/profile')

You can specify the request type in the request header.

    headers = {'Content-Type': 'application/x-www-form-urlencoded'}
    response = requests.post(endpoint, data=request_body, headers=headers)
    print(response)

You don't need to explicitly encode it, simply pass your dict to data argument and it will be encoded automatically.

>>> r = requests.post(URL, data = {'key':'value'})

From the official documentation:

Typically, you want to send some form-encoded data — much like an HTML form. To do this, simply pass a dictionary to the data argument. Your dictionary of data will automatically be form-encoded when the request is made