How to have iTunes Windows 10 backup to external drive?

you find the dir it stores backups in on your win10

Locate backups of your iPhone, iPad, and iPod touch - Apple Support

then you mount the external hd as that dir

How to mount hard drive as folder on Windows 10 | Windows Central

and bobs your uncle

more guides

windows 10 mount external hd as dir - Google-

You can't do that using iTunes, sorry.

If you can, move docs or other media from your internal hard disk to an external HDD or SSD to make room, so that you can use iTunes.

or

Use a 3rd party app (i.e. iMazing) to backup directly to a different location.

Regards

Giulio