I want the option on my Apple tablet Pro to limit the charging to 85% power…….

There is no such option. Your iPad will manage its own charging. Just plug it in a leave it/use it as normal.

While Optimised charging can be enabled/disabled on iPhone, an equivalent setting doesn’t exist for iOS/iPadOS for iPad. iPad optimised charging for iPad is always active and will optimally charge the iPad battery - but differs from iPhone in that full charging to 100% is not delayed.  

Your iPad will automatically regulate and optimise charging - and will not overcharge the battery. When connected to a Power Adapter for an extended period of time, such as when used in Kiosk or PoS (Point if Sale) applications, iPad will change its charge management strategy - and reduce the maximum battery charge level to about 80% of maximum capacity. This modified charging strategy is designed to reduce chemical ageing that might otherwise occur if 100% charge is maintained at all times.

Nominal 100% charging will be restored when a normal charge/discharge routine is restored.