To link multiple Assembler modules where the calls reference entry points in the called programs rather than their external names, you need to:
You can find this check box on the Link page of the Property Pages for the module.
You can find this checkbox on the Compiler tab on your project properties' Assembler page.
If you compile individual modules, you must select the .lin file for compilation too. If you build or rebuild the whole project, the .lin file is compiled with the other source files but will always be linked after any assembly to ensure that only the latest .OBJ files are linked. In either case, the Assembler Linker looks in the .lin file to find which object files need to be included in the executable .390 or .MOD file.