A valid web manifest file is one of the primary PWA requirements, but you would be surprised at how many sites miss this simple step. Most of them just forget an important field or to supply values that don't meet the guidelines, such as a long and short name value.
The biggest mistake is not including the minimum set of icons required by all the different browsers and operating systems.
To use the online tool, you need to supply a public URL, two primary application colors, and a logo or image you want to make application icons:
The wizard will try to parse values to supply properties, such as full name, ...