I'm not sure I understand exactly what you're after, but there are a number of variables that can be used in External launch configurations.
From your description, the likely one is to put ${resource_loc} in the Working Directory: field.
From the description:
Returns the absolute file system path of a resource. The target resource is the selected resource when no argument is specified, or the resource identified by a workspace relative path.
Alternatively, ${container_loc} might be more appropriate if you need to run on a directory:
Returns the absolute file system path of a resource's container. The target resource is the selected resource when no argument is specified, or the resource identified by a workspace relative path.