Hi there 
There is Ash.PlugHelpers.set_tenant/2.
The thing to note is that the Ash plughelpers are there to provide a consistent interface across various extensions that interact with plugs - ie AshJsonApi, AshGraphQl and AshAuthentication. Note that these won’t do anything for your code - you would still need to call get_tenant/2 and pass it into your actions as an option.
There is also Ash.set_tenant/1 which sets it in the process dictionary but that feature is going away in 3.0 on favour of requiring it to be explicitly passed as above.
