Activate ASM policy via REST API

In v12 you will need to run this as a task.

Find the status and the self link of the target policy:

$ curl -sku admin:admin https://155.121.lab.es.f5net.com/mgmt/tm/asm/policies/Fqpvw5rn1SzYqgrCR9CA1Q | jq  '.active, .selfLink'
false
"https://localhost/mgmt/tm/asm/policies/Fqpvw5rn1SzYqgrCR9CA1Q?ver=12.1.3"

Using the selfLink from the previous step start the task:

$ curl -sku admin:admin -X POST -d '{"policyReference": { "link": "https://155.121.lab.es.f5net.com/mgmt/tm/asm/policies/Fqpvw5rn1SzYqgrCR9CA1Q?ver=12.1.3"}}' https://155.121.lab.es.f5net.com/mgmt/tm/asm/tasks/apply-policy | jq .
{
  "kind": "tm:asm:tasks:apply-policy:apply-policy-taskstate",
  "selfLink": "https://localhost/mgmt/tm/asm/tasks/apply-policy/42ZU2wkHI2BLOCx6VYNsVA?ver=12.1.3",
  "policyReference": {
    "link": "https://localhost/mgmt/tm/asm/policies/Fqpvw5rn1SzYqgrCR9CA1Q?ver=12.1.3"
  },
  "status": "NEW",
  "id": "42ZU2wkHI2BLOCx6VYNsVA",
  "startTime": "2018-06-05T15:22:25Z",
  "lastUpdateMicros": 1528212145000000
}

Validate the task has completed:

$ curl -sku admin:admin https://155.121.lab.es.f5net.com/mgmt/tm/asm/tasks/apply-policy?\$filter=id%20eq%20\'42ZU2wkHI2BLOCx6VYNsVA\' | jq .
{
  "kind": "tm:asm:tasks:apply-policy:apply-policy-taskcollectionstate",
  "selfLink": "https://localhost/mgmt/tm/asm/tasks/apply-policy?ver=12.1.3&$filter=id%20eq%20'42ZU2wkHI2BLOCx6VYNsVA'",
  "totalItems": 1,
  "items": [
    {
      "status": "COMPLETED",
      "lastUpdateMicros": 1528212157000000,
      "selfLink": "https://localhost/mgmt/tm/asm/tasks/apply-policy/42ZU2wkHI2BLOCx6VYNsVA?ver=12.1.3",
      "kind": "tm:asm:tasks:apply-policy:apply-policy-taskstate",
      "policyReference": {
        "link": "https://localhost/mgmt/tm/asm/policies/Fqpvw5rn1SzYqgrCR9CA1Q?ver=12.1.3"
      },
      "endTime": "2018-06-05T15:22:38Z",
      "startTime": "2018-06-05T15:22:25Z",
      "id": "42ZU2wkHI2BLOCx6VYNsVA"
    }
  ]
}

Validate the policy has been activated:

$ curl -sku admin:admin https://155.121.lab.es.f5net.com/mgmt/tm/asm/policies/Fqpvw5rn1SzYqgrCR9CA1Q | jq  '.active, .selfLink'
true
"https://localhost/mgmt/tm/asm/policies/Fqpvw5rn1SzYqgrCR9CA1Q?ver=12.1.3"

Please let me know if this addresses your question.