If you want to manually generate project files with a precompiled engine (acquired via Epic Games launcher, not Source) you can invoke UBT directly with the following arguments:
"%ProgramFiles%\Epic Games\UE_4.22\Engine\Binaries\DotNET\UnrealBuildTool.exe" -ProjectFiles -UsePrecompiled -Game "absolute\path\to\your\project\file.uproject"
(Replace the paths to the engine and project file with your own paths.)