Sorry, I'm still a bit confused.
You have to specify values because that's what the percentile_ranks aggregation does: it tells you the rank of specific values that you care about.
If you just want to know the 90th percentile and find out what the value is at that point, you should use the percentile aggregation instead.
Basically, percentile_ranks is for asking what the rank of specific, known values is. percentiles is for asking what value is greater than n percent of the data.