The Board of Selectmen on Tuesday will host a public forum to discuss a $200 million slots parlor proposed for 30 acres off Route 133 in neighboring Tewksbury. The forum will be held at 7 p.m. in Memorial Hall Library. John Ziembra, ombudsman for the Massachusetts Gaming Commission, is expected to discuss the state’s gambling law, particularly a provision that requires a casino company to negotiate the effect of its development with surrounding communities. Penn National Gaming, which proposes to build a slots parlor in Tewksbury near the Andover line, last week designated Andover a surrounding community. “There is a serious concern about traffic, public safety, and the overall impact of that facility would have on our town,” said Andover Selectman chairman Alex Vispoli.