Adding variables to an API payload with new lines in Python

I am pretty new to Python, and what I'm trying to achieve is to add variables to the payload below in Python; it's meant for Zeptomail API, and I'm trying to customize and add user-specific data to the payloads 'merge info' and the 'to' header before sending the email template.

example.py

test_name = 'users name'
test_email = 'users@email.test'
test_title = 'users title'

payload = '''
    {
        \n\"mail_template_key\":\"xxxxxxxxxxxxxxxxxxxxxxxxx\",

        \n\"from\": {
            \"address\": \"example@mail.com\",
            \"name\": \"example\"
            },

        \n\"to\": [{
            \"email-address\": {
                \"address\": \"test_email\",
                \"name\": \"test_name\"
                }
            }],

        \n\"merge_info\": {
            "name":"test_name",
            "title":"test_title"
            }}
'''

I'm expecting to print out a new payload variable with the included user-specific, 'test_' values in place.

expected print(),

payload = '''
    {
        \n\"mail_template_key\":\"xxxxxxxxxxxxxxxxxxxxxxxxx\",

        \n\"from\": {
            \"address\": \"example@mail.com\",
            \"name\": \"example\"
            },

        \n\"to\": [{
            \"email-address\": {
                \"address\": \"users@email.test\",
                \"name\": \"users name\"
                }
            }],

        \n\"merge_info\": {
            "name":"users name",
            "title":"users title"
            }}
'''

The payload is JSON-formatted, so simply build a Python object of dicts/lists, then convert to a JSON string:

import json

test_name = 'users name'
test_email = 'users@email.test'
test_title = 'users title'

data = {'mail_template_key': 'xxxxxxxxxxxxxxxxxxxxxxxxx',
        'from': {'address': 'example@mail.com',
                 'name': 'example'},
        'to': [{'email-address': {'address': test_email,
                                  'name': test_name}}],
        'merge_info': {'name': test_name,
                       'title': test_title}}

payload = json.dumps(data, indent=2)  # indent for pretty-printing, an API doesn't need it.
print(payload)

Output:

{
  "mail_template_key": "xxxxxxxxxxxxxxxxxxxxxxxxx",
  "from": {
    "address": "example@mail.com",
    "name": "example"
  },
  "to": [
    {
      "email-address": {
        "address": "users@email.test",
        "name": "users name"
      }
    }
  ],
  "merge_info": {
    "name": "users name",
    "title": "users title"
  }
}

If you need to update the existing payload string you have, convert it from JSON, patch the fields, and convert it back to JSON:

import json

test_name = 'users name'
test_email = 'users@email.test'
test_title = 'users title'

payload = '''
    {
        \n\"mail_template_key\":\"xxxxxxxxxxxxxxxxxxxxxxxxx\",

        \n\"from\": {
            \"address\": \"example@mail.com\",
            \"name\": \"example\"
            },

        \n\"to\": [{
            \"email-address\": {
                \"address\": \"users@email.test\",
                \"name\": \"users name\"
                }
            }],

        \n\"merge_info\": {
            "name":"users name",
            "title":"users title"
            }}
'''

data = json.loads(payload)
data['to'][0]['email-address']['address'] = test_email
data['to'][0]['email-address']['name'] = test_name
data['merge_info']['name'] = test_name
data['merge_info']['title'] = test_title

payload = json.dumps(data, indent=2)
print(payload)  # same output as above