From John:

curl -s -X PATCH \
  "https://api.openshift.com/api/assisted-install/v2/clusters/997890e5-55e3-41c2-8d5c-f3aa4a8a82b1/install-config" \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $API_TOKEN" \
  -d '"{\"fips\":true}"'

Ticket needs more scoping.