If a value isn’t likely to change over time, or between environments or datacenters, then don’t put it into configuration. Use a constant instead.
<add key="ssrPageUrl" value="/{0}pages/agoda/default/DestinationSearchResult.aspx?asq={1}" />
public const string SSR_PAGE_URL = "/{0}pages/agoda/default/DestinationSearchResult.aspx?asq={1}";