Craft the Necrozma Dusk Radar to locate the tower, or search for it in The End

After combining the radar with an empty map in The End in an End Cartography Table, you should be able to find this structure.

At the entrance to the tower, you will see a pressure plate. It will make you levitate you to the top. Make sure you line yourself up properly, the margin for error is slim. If you miss the opening, you will fall, so you could also bring blocks to stop your fall and then platform up. This is the same as the Dawn Tower

Step on the pressure plate to spawn Necrozma Dusk Mane.