Answer:
The forklift cylinder should have a 3/4 inch vapor outlet in it. A vapor valve can be installed in this outlet although a fill valve will usually need to be removed first. The cylinder can be filled and used through the vapor valve. Never try to hook up the liquid port that is used for a forklift to a grill. Or take the forklift cylinder to a propane dealer do to the high cost of forklift cylinders a used forklift cylinder is worth more than a new grill bottle.
If you are highly experienced in piping flammable gases, you may be able to connect the fuel tank of a propane fork truck to your grill. This is a very dangerous thing to do and should not be attempted by any but the most experienced people.