How do manual licensing for unity 3d?
Unity should generate a file which you upload here: https://license.unity3d.com/
Where do I find that file?
Unity3d > Help > Manage license > Manual activation , have fun 
The rest is obvious.
Unity does not load up it. It says there was a problem with the license server. So I need to know how to save and manually submit the license?
Follow the instructions here: Manage your license | Unity Hub | 3.0