# Sheet name without spaces or special characters
=SheetName.A1

# Sheet name with spaces or special characters
='Sheet name'.A1
# or if you wath to make sure the sheet name does not change
=$'Sheet name'.A1

# src: https://help.libreoffice.org/3.3/Calc/Referencing_a_Cell_in_Another_Document
# src: https://ask.libreoffice.org/en/question/170921/calc-need-syntax-for-sheet-with-a-space-in-its-name/