Hi Ines,
You should use the SalesOpportunities instead of PurchaseOrder then you can link your created document to the opportunity, your code will look like this
SAPbobsCOM.SalesOpportunities sales = (SAPbobsCOM.SalesOpportunities)oCompany.GetBusinessObject(SAPbobsCOM.BoObjectTypes.oSalesOpportunities);
if (sales.GetByKey(1))
{
sales.Lines.SetCurrentLine(sales.Lines.Count - 1);
sales.Lines.DocumentNumber = 67;
sales.Lines.DocumentType = SAPbobsCOM.BoAPARDocumentTypes.bodt_PurchaseOrder;
sales.Lines.Add();
}
int error = sales.Update();