How do I attach “LibreOffice” to the ‘Open with’ drop down in ‘Windows 10’?
A question in return: Why do yopu want to get LibreOffice under ‘Open With’ instead of associating it with its own standard extensions once and for all by the install process?
This fix association should be the recommended proceeding.
For the rare cases where you may want to experimentally open and ODF type files with a different software, you may then go the long way.
“It is not good that ODF files be opened with alien and probably even hostile software.”
Go once the long way
(Roughly and by guessing translated from my silly German localisation of Win 10 to English. This locale hype is really a mess gravely endangering international coopration. It should be abandoned.)
Open With... > (to bottom) > Select different App > Different Apps > (to bottom) > Additional Apps > select the appropriate folder for LibO from 'Program Files' or 'Program Files x86' > enter folder 'program' > select 'soffice.exe' > OK
If you didn’t check the ‘Always use this App to open … Files’ the LibO will be added to the ‘Open-With’-list.
If you did check the mentioned option the file extension will be fix associated with the ‘Open’ command.
You need to do this for each extension you want to put into the Open-With-list.
There surely are different and probably simpler ways to the same effect.