Declaring a retrofit REST endpoint with constant query value

In kotlin you can specify the default parameter:

interface YoutubeApi {

    @GET ("oembed") 
    suspend fun metaData (
        @Query (QUERY_VIDEO_URL) url: String,
        @Query(QUERY_FORMAT) alwaysJson: String = "json"
    ): Response<YoutubeMetaData>

}

Just put it right in the relative URL:

public interface YoutubeApi {
  @GET("oembed?format=json") 
  YoutubeMetaData metaData(@Query(QUERY_VIDEO_URL) String url);
}