Requires ASA 9.3(1) or higher which requires the 5500-X line or ASAv.
.spa file from cisco.com. Place it on the ASA. Then enable ASDM and the rest-api.
You can view documentation of the API and test that the API is working by going to the following URL (replace the IP with your ASA IP).
To properly view the documentation, your computer needs access to the internet to download specific resources like jquery.