Hello,
I saw the following answer on the insider forum:
The On-Prem version does have an API for the helpdesk element, and for asset information you can connect directly to the SQL DB. There are no plans currently to add an Asset API to the on-prem version.
It may not be exactly what you are looking for but the ADP cloud version does sync all your assets from on-prem. So it will be possible to get API access to your on-prem assets via the ADP API.
So it seems that there will be no API for the on-prem version. The documentation is for the cloud based version.
This is the closest thing I found: https://github.com/MarcoGriep88/API-Lansweeper (I haven't tried this)
Hope I could help.