How can I use "source" dimension to analyze a specific event in Google (Firebase) Analytics?

Question

I'm using Google (Firebase) Analytics in an iOS app and track the event like follows.

Analytics.logEvent("event-name", parameters: [
    AnalyticsParameterSource: "source-value"
])

AnalyticsParameterSource is pre-defined in Firebase SDK as follows.

/// airship). Identify the advertiser, site, publication, etc. that is sending traffic to your
/// property. Highly recommended (String).
/// <pre>
///     let params = [
///       AnalyticsParameterSource : "InMobi",
///       // ...
///     ]
/// </pre>
static NSString *const kFIRParameterSource NS_SWIFT_NAME(AnalyticsParameterSource) = @"source";

It can be confirmed that the "source" is logged as expected when I see the event detail page in GA.

However, if I try to use the "source" dimension in Explore feature in GA, it doesn't show up in the dimension candidates even if I search "source" there. Is there any special step to use the "source" dimension in GA?