Call activity tenant shared definition

Is there a way to set the tenant id on the call activity to explicitly go to the shared definition.

I saw that you can specify a tenant, or a function to determine the tenant, or leave it empty.
But I didn’t saw a way to explicitly set it to shared definition.

It is possible to use a function to return tenant for private definition, or null for shared definition. But I would like to avoid that if possible.

I’ve tried ${""}, ${null} as values. And it doesn’t work.

Suggestions are appreciated.
Meanwhile, if I find a solution a will update the post.

Update: ${null} is working, I had some issues on my project.