You should find that you can simply create a tam-tam instrument in your score, then in Play mode, set the Routing section to point at the appropriate channel in Kontakt, where you’ve loaded the patch containing the tam-tam sound. Then the only remaining thing is to tell Dorico which MIDI pitch it needs to play to trigger the tam-tam sound in Kontakt. To do that, you’ll need to create a simple percussion map (via Library > Percussion Maps) that specifies the tam-tam instrument, the Natural playing technique, and the MIDI pitch that corresponds to the tam-tam sound in your Kontakt instance. Finally, using the Endpoint Setup dialog, assign your new percussion map to the Kontakt channel with the tam-tam sound loaded. It sounds more complicated than it is.
If you want a more detailed tutorial for how percussion mapping works, read this: