I have noticed that there is no keyboard shortcut for inserting a horizontal line in LibreOffice Writer. I tried to go to Tools->Customize to add my own but under “Insert” there is no option to select “Horizontal Line”. Why is this the case?
Same problem after install of 6.3.0.4, the previous method listed by Mike Kaganski was no longer working. I have succeeded with the auto-correction option answered by JBF. Quite obscure!
Type this:
Enter→---→Enter.
Note: you have to turn that on in Autocorrect options.
In v7. & earlier 6.4 you may use — to create a horizontal line.
But that is a pesky troublesome thing.
It appears to change the paragraph style at the above paragraph to alt+o +a (format paragraph) select tab “border”, click in box to create user defined, “underline paragraph”.
If you aren’t aware of this, as I so tediously learned… trying to get rid of your “—” is maddening.
The more sensible “horizontal line” htlm= <hr /> just isn’t in the program it appears. BUT… when you use alt+i L “insert horizontal line” what appears to happen is the paragraph above where you want your horizontal line is changed to “paragraph style” “horizontal line”. I believe this is exactly the same as the single line created by “—” but has been pre ordained to be a paragraph style like “default style” “Subtitle” etc.
It is the double line instead of the single line from underline border to the paragraph.
Both solutions to insert <hr /> a horizontal line seem insane to me. You can insert nearly anything but an actual horizontal line. 
pray for a solution. But no actual <hr /> I can find.
Additionally, to undo one of these monsters turn on ¶ (see hidden characters), select the ¶ above the horizontal line and in the formatting toolbar (click on) change the format to default style or whatever style you want before the "not <hr />" insertion.
Took me long enough to figure it out. Hope this helps & saves someone an hour or two, providing they find the answer… or even know where to look for it. Yours Truly 
Sagely Fox.
I am using an older version of LO, ver 6.1.5.2 but I wonder if the method I have always used would work with these latest versions. I make my own version of it for some reason, I think because I don’t want the Style to change to whatever it changes to after you insert a Horizontal Line. Ya, just looked at the style properties of Horizontal Line, and it is set up under the Organizer tab to change to text body as the Next Style. So I create my own horizontal line so as to not disturb the properties of the original, and mine will be Default Style as the next style or following style, whatever.
Anyway, just fixed mine up again and assigned it to a keyboard shortcut. I assign mine to Alt+L, but you can use whatever you want. To do a quick test try, why don’t you try this.
- Click Tools > Customize > and click on the Keyboard tab
- In the top box, labeled Shortcut Keys, scroll down to whatever shortcut you want to use and click on it one time. Like I said, I like to use alt+L
- Then, there are 3 boxes at the bottom, Category Function and Keys
- In Category box, scroll down the list to the bottom, where it says Styles (which has one of those drop-down list arrowheads next to it) and click on that drop-down list arrowhead on the left. It will open a drop-down list of Styles. You want to single-click on the one named Paragraph.
- That will bring up a list in the middle box, Function. Scroll down and you should see Horizontal Line, in my older version it is just after you get past the Heading entries, Heading 10 is the last one on my version. Single click on Horizontal Line.
- Now, go up to the buttons on the right, Modify Load Save Delete Reset. Single-click on Modify. You should see Horizontal Line appear next to whatever shortcut key you picked in the Shortcut Keys box up top, and in the Keys box in the bottom right, it should now show that same shortcut key.
- Without doing anything else to mess up your new set-up, click on OK button. I think the shortcut key is now automatically assigned, and the window closes and you are good to go.
Hopefully this still works in the new versions, like it has done in all the old past versions I have used. If not, at least I gave it a try. If you try it with the latest versions and it works, let me know if you want to know how to make a new ‘custom’ Horizontal Line Style so that you don’t have to make any changes to the ‘default’ one that came with LO. It is an easy process too. Hope this works for everyone.