Two-key start-end workaround:
- Press 
F8 (to switch to Extending selection mode)¹ 
- Press 
Ctrl+Shift+B² 
- Type the subscript numbers
 
- Press 
F8 (to return to Standard selection mode) 
Three-key workaround (this way you don’t need to remember to get out of Extending selection mode):
- “Select nothing” (that is: hold 
Shift and press Left Arrow and Right Arrow, or Right Arrow and Left Arrow)³ 
- Press 
Ctrl+Shift+B² 
- Type the subscript numbers
 
¹ Or choose Extending selection in the status bar.
² Or the shortcut for the desired text format.
³ Up Arrow and Down Arrow, or Down Arrow and Up Arrow, will work if not in the first or last line (respectively) of the document.
Tested with LibreOffice 7.0.6.2 (x64); OS: Windows 10.0.
Add Answer is reserved for solutions. If you think the answer is not satisfactory, add a comment below, or click edit (below your question) to add more information. Thanks.
Check the mark (
 → 
) to the left of the answer that solves your question (it may take a little while until it turns green).
If the answer helped you, you can mark the up arrow (
) that is on the left (to vote, you need to have karma of at least 5).