If you download the test project from GitHub and open it in GMEdit instead of GM, you can see that the extension contains three files - execute_shell_simple_ext.dll, execute_shell_simple_ext.gml, and autogen.gml. Right-clicking the extension and picking “Show API” will show you which functions go in which
If you double-click the extension in 2023.11 IDE, however, you can see that the file names got eaten - you’ll need to double-click on each “mystery file” in the list and assign it the correct name-extension.