How to access existing variable with Data Store Read in Simulink?

I use a third party c library with my Simulnk model as Custom Code. I'd like to access a global variable exported in this library with a Data Store Read block from the Simulink model. However I don't know how to tell the Data Store Memory block that it is an already existing variable. It always exports the definition no matter what, and also initializes the variable with de default values. How can it be achieved? Thanks!