I've found one website that obviously seems to have managed this task: https://go2.travel-dealz.de/. A search there creates automatically the correct Google Flight hyperlink.

Since my knowledge in this field is very limited, I am only interested in a solution to create such links by using Google Spreadsheet. For a Oneway-flight this would mean: Enter departure, destination and date in cells A1, A2 and A3, and then get the encoded link in A4.