Create new vendor invoice by X++ code using FormRun

Hi everyone,

I am trying to create a new vendor invoice in creation mode using this code : 

PurchFormLetter_Invoice purch;
Args args;
FormRun run;

purch = PurchFormLetter::construct(DocumentStatus::Invoice);

purch.parmOpenMode(OpenMode::New);

args = new args(formStr("VendEditInvoice"));

args.caller(purch);

run = classFactory.formRunClass(args);

run.init();

run.run();

run.detach();

The form is not opening because there seems to be missing a parmId in the init() method of VendEditInvoice...