I checked the samples/API references here:
- https://msdn.microsoft.com/en-us/microsoft-teams/connectors
- https://dev.outlook.com/Connectors/GetStarted
- https://dev.outlook.com/Connectors/Reference
But cannot find how to mention someone in a new message to a channel.
Programmatic @mentions are not currently supported by Microsoft Teams, and will probably not come soon.
There is such a wish from the community in the MS uservoice portal, but it seems not to burn as much as other issues: