How to format/indent single-line XML files in Notepad++ x64?

Download the latest 64-bit x64 (stable) release of Notepad++ here: https://notepad-plus-plus.org/download/ (version 7.4.2 at the time of writing) and start the usual install.

Download the latest 64-bit x64 (stable) release of NppPluginManager here: https://github.com/bruderstein/nppPluginManager/releases

Unzip downloaded archive in the N++ directory (e.g. C:\Program Files\Notepad++) and run N++

Go to the menu Plugins > Plugin Manager > Show Plugin Manager. Wait for the list to refresh and scroll down to find the XML Tools plugin (at the time of writing this, it's version 2.4.9.2). Check the corresponding box and click Install.

Wait for the install to complete, then once prompted, choose to restart N++.

[ Maybe you'll have to confirmed that you want to copy/use some dlls anyway (like libcurl.dll and libxml2-2.dll - don't know why) ]

Then you're done: go to the Plugins menu then XML Tools and you'll find your usual commands you were familiar with in TextFX.

I had the same issue with Notepad++ v 7.6 - could not install XML Tools. Updated the N++ to v 7.6.3 and after that XML Tools could be found in Plugin Manager's collection. Now everything works all right.