r = requests.Session()
theaders = {
        'accept':   '*/*',
        'content-type': 'application/x-www-form-urlencoded',
        'accept-encoding':  'br, gzip, deflate',
        'user-agent':   'mobile_mrp_mrporter/6.8.6 (iPhone; iOS 11.2.1; Scale/3.00)',
        'accept-language':  'en-US;q=1',
        'content-length': '58'

    }

    tdata =  "j_password=password&j_username=user@gmail.com"
    test = r.post('https://www.mrporter.com/am/api/auth/login.json',data = tdata,headers=theaders)

{
    "response": "SIGNED_IN"
}