you can't upload your own server jar, we'll check if the file matches what was set in the basic settings. If it does not match, the file will be overwritten.
We do this to increase security. Malicious people could otherwise upload and run malicious java applications.
Therefore, the Custom Modpack is called "Custom Modpack (Outdated & Broken)" as it does not work with this technology.
We'll discuss tomorrow if we'll add the software to the library and then answer you again