According to Zeenews, the billboard will be outside of a CubeSat [a small satellite about the size of a milk carton] which is a cheaper and faster alternative to traditional satellites.The billboard will be around 8x8 cm big and will have 400 squares available for purchase. It will orbit the Earth 15 times a day for 10 years.
You can go to the billboard website and see the messages or logos that are on the actual satellite because the billboard will not be seen from Earth.
It's not just the companies who will be able to put a message on the billboard but anyone can write a message but that will cost 1 euro a character.
The CubeSat carrying the billboard will focus on global warming research once it is in space. The money sponsors invested on the billboard will be used to expand research on small satellites at the KU Leuven University.