POST /WS/OpenAPI.asmx HTTP/1.1
Host: bms.kaseya.com
Content-Type: text/xml; charset=utf-8
Content-Length: length
SOAPAction: "https://bms.kaseya.com/Authenticate"

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <Authenticate xmlns="https://bms.kaseya.com/">
      <companyName>string</companyName>
      <userName>string</userName>
      <password>string</password>
    </Authenticate>
  </soap:Body>
</soap:Envelope>

import requests

companyName = 'My Company'
userName = 'My Username'
password = 'My Password'

url = 'https://bms.kaseya.com/WS/OpenAPI.asmx'

body = '''
<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <Authenticate xmlns="https://bms.kaseya.com/">
      <companyName>{}</companyName>
      <userName>{}</userName>
      <password>{}</password>
    </Authenticate>
  </soap:Body>
</soap:Envelope>'''.format(companyName,userName,password)

headers = {'Host':'bms.kaseya.com',
           'Content-Type':'text/xml; charset=utf-8',
           'Content-Length':str(len(body)),
           'SOAPAction':'"https://bms.kaseya.com/Authenticate"'}

r = requests.post(url, data=body, headers=headers)
print(r.text)

<?xml version="1.0" encoding="utf-8"?><soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><soap:Body><soap:Fault><faultcode>soap:Client</faultcode><faultstring>Server did not recognize the value of HTTP Header SOAPAction: https://bms.kaseya.com/Authenticate.</faultstring><detail /></soap:Fault></soap:Body></soap:Envelope>