Add a module dependency to a sandboxed project
I am afraid that is not possible. 

One dirt-fix would be to add the following in your info file.

dependencies[] = sandbox/mihai_brb/2254845


This will make sure that users will not be able to enable this module by default and cause issues. And then you can add the instructions in your module page and README.txt you can add the instructions about the dependencies and how to download them and then remove the line from the info file.

A better approach would be to use hook_requirements and in phase = "install" you can check for the presence of a function that is present in the sandbox module. If that function is not present, you can block the module installation and show a message to the user to download the sandbox module and then try installing again.