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 $45\mathrm{\%}$‍ of voters, so the manager wants to test $H_0:p=0.45$‍ versus $H_{\text{a}}:p<0.45$‍, where $p$‍ is the proportion of voters that support the opposing candidate.

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