A clematis (large flowered type which you would prune back to 9in (20cm) every February, or try a perennial sweet pea, Lathyrus latifolius. The soil at the base of the bush is likely to be poor so make the planting hole about 2ft (60cm) from the Forsythia. Fill it with some Tree, Shrub & Rose Planting Compost and Bonemeal. Train the climber along twiggy sticks and into the bush.