It seems that you misunderstood SEOstats' documentation, as well as SEMruch's free organic keyword reports.
There is no way to request data for specific, user-defined keywords with the public SEMrush API. Instead, they provide a list of relavant keywords related to a specific URL (eg "to find long-tail keywords").
So, you can feed $SEOstats->SEMRush()->getOrganicKeywords()
with a URL, and get relevant keyword metrics for that URL in return. And there is no option for user-defined keywords.