BIG-IP iControl REST 12.1.2 documentation

Here's how I did it in Python. I couldn't find the answer anywhere, and finally figured it out. Having to do a 'patch' and the payload is {"createDraft":"True"} is the magic.

Publishing the draft is also a little tricky. I've included a code snippet for that too.

import requests

lb_name = "f5"
username = "admin"
password = "admin"

def create_policy_draft(policy_name):
    ret = requests.patch(
        'https://%s/mgmt/tm/ltm/policy/%s' % (lb_name, policy_name),
        json={"createDraft": True},
        auth=(username, password),
        verify=False
    )

def add_policy_rule(draft_policy_name, rule_json):
    ret = requests.post(
        'https://%s/mgmt/tm/ltm/policy/~Common~Drafts~%s/rules' % (
            lb_name, draft_policy_name),
        json=rule_json,
        auth=(username, password),
        verify=False
    )

def publish_policy_draft(draft_policy_name):
    ret = requests.post('https://%s/mgmt/tm/ltm/policy' % lb_name,
                        json={"command": "publish",
                              "name": "Drafts/%s" % draft_policy_name},
                        auth=(username, password),
                        verify=False)