Filippo Moyersoen of Italy entered the Capital Centre ring at the Washington International Horse Show last night for the $10,000 Puissance high jump class riding a horse named Adam. He left the ring driving the new Mercedes Benz he won after setting a North American indoor high jump record of 7 feet, 6 inches.

The previous record of 7-5 was set by Barney Ward aboard Lucky Hit at the 1978 Royal Winter Fair in Toronto. Ward of Brewster, N.Y., placed second in last night's event that went to four jump-offs. Ward, aboard Glandor Akai, had to settle for second after the French-imported horse failed to negotiate the last of the three fences that remained. A crowd of more than 11,000, second highest in the history of the show, looked on.

William Martin of Alvin, Tex., who shared the winner's circle with Ward last year when the yellow simulated brick wall went up to 7-4, placed third on Hole In One.

Moyersoen took a victory lap in his new $20,000 car. According to a show aide, his first question on getting out of the car was, "How do I get this to Italy? "

Moyersoen, 27, lives in Milan. He said he never has ridden his 10-year-old English-bred gelding in this type of high jump competition. His coach, Raymondo D'Inzio, a world championship rider himself, said Adam twice before had jumped 7 feet and once cleared 7-2. The world indoor record is 7-7 5/16 set in 1979 by Nick Skelton of England on Everest Lastic. The outdoor mark of 8-1 1/2 was set in 1949 by Alberto Larraguibel of Chile riding Huaso ex-Faithful.

Moyersoen said he was confident about the record-setting jump. "I was tranquil about it. The horse was calm enough. I wasn't worried about it that much."

Adam seemed more upset by the standing ovation after the record was set and was too nervous to accompany his rider back in the ring for the award ceremony.

After Ward had failed to clear the 7-6 height, he informed the announcer that he would be willing to jump against Moyersoen if the Italian had also failed to clear the wall.

The challenge became moot when Moyersoen and Adam cleared the wall, just edging the rounded top. It was jarred slightly out of place but there never was any danger that it would fall off.