I recently needed to set currency for a record in CRM in my plugin. It sounded a good piece of code to refer to my blog as it is often needed. Here it goes:
EntityReference currencyType = new EntityReference();
currencyType.Id = “(The Guid Of The Currency Type Goes Here)”;
currencyType.LogicalName = “transactioncurrency”;
For JS reference refer my older blog post: Click here
Hope it helps!