Calculating the P-value in a z test for a proportion

A campaign manager for a political candidate released a series of advertisements criticizing the opposing candidate in an upcoming election. The opposing candidate previously had the support of $45\%$45, percent of voters, so the manager wants to test $H_0:p=0.45$H, start subscript, 0, end subscript, colon, p, equals, 0, point, 45 versus $H_\text{a}:p < 0.45$H, start subscript, start text, a, end text, end subscript, colon, p, is less than, 0, point, 45, where $p$p is the proportion of voters that support the opposing candidate.

After running the advertisements, the campaign manager obtained a random sample of $500$500 voters and found that $200$200 of those sampled supported the opposing candidate.