Problem accessing outputs from another service

After I update my version of the serverless framework, I’m getting this warning when trying to access outputs from another service:
Variable syntax error at “provider.iam.role.statements.0.Resource.1”: Invalid variable address at index 2 in "{output:app-name:{self:custom.stage}:us-east-1:service-name.out-variable}
Before the update it worked normally, I have been trying to find out how is the new variable syntaxe, but I could not find it anywhere.
The page stating the cause of the warning is not much help. (https://www.serverless.com/framework/docs/deprecations/#NEW_VARIABLES_RESOLVER)

Can someone tell me how is the new syntaxe to access outputs from another service?