If you need a build the Version Installer does not list, you can upload the jar yourself.
1. Stop the server.
2. Upload the jar to the main folder on the Files page, next to the existing one. For anything above about 100 MB, SFTP is more reliable than the browser uploader.
3. Tell the server to use it. The Startup tab has a field for the jar file name. Set it to your file name exactly, including the .jar ending. The server runs whatever is named there, so a typo means it will not start.
4. Start and read the console. The first lines tell you which build actually loaded.
The jar has to match your Java version. This is the most common reason a custom jar fails. An error mentioning UnsupportedClassVersionError or "compiled by a more recent version of the Java Runtime" means the build needs a newer Java than the server is set to — tell us which Minecraft version the jar targets and we will set the matching Java for you.
Keep the original jar in place rather than deleting it. If the custom build misbehaves, switching the name back is the fastest way to get running again.
If you need help with this, please submit a ticket.