The tape usually indicates the start of the winding, ie which end of the wire was affixed to the bobbin first during the transformer winding process.
When connecting the transformer for a 240v supply, the two primary windings must be connected in series.
The phasing of the two windings is very important, the correct way works fine, the wrong way lets out the magic smoke*.
The correct way to connect the windings is to connect the taped end of one winding to the untaped end of the other winding.
You will then have a taped end and an untapped end to connect to the switch/fuse etc of the incoming mains supply, as per the schematic diagram.
Looking at the schematic, you will see that the primary windings of the transformer have been (ambiguously) annotated with big black dots.
When correctly drawn, the dots should be adjacent to the wire (not on the wire), the dots indicate the start of the winding.
I hope that this clears your confusion.
JimB
* All electronics works using magic smoke.
When the magic smoke comes out, the electronics stop working.