Question

How to add a new line character (the same when the Enter key is pressed) to the Merge Fields rule?

Answer

Via the Merge Fields rule it is not possible, however, the fields could be merged via a script rule:

//C# sample
var date = Context.Field("InvoiceDate").Text;
var num = Context.Field("InvoiceNumber").Text;
Context.Field("test").Value = date + "\r\n" + num;

Make sure that the needed fields are added to the Available Fields list in the rule settings and for the result field the Read Only option is disabled: