select ExpenseCategory

I'm creating a new expense, I can create it successfully, but I can't define its category, I can't find anything, no example in the API documentation or in the forum so far that helps me.

Follows or works so far, just add a line that selects a category for expense.

"{'entity':{" + "'Id':'" + expense.getId() + "',"
+ "'Description':'" + expense.getDescription() + "',"
+ "'DateIncurred':'" + expense.getDateIncurred() + "',"
+ "'LocalAmount':{"
+ "'currency':'"+ expense.getcurrency() + "',"
+ "'value':'" + expense.getValue() + "'},"
+ "'ExpenseSheet':'" + idExpenseSheet + "',"
+ "'Category':'"+ expense.getCategory() +"'}"
+ "'fields':['CreatedOn','ExternalId','Description','Category','ExpenseSheet']}",

Thanks.