How to use an absolute reference when extending a formula sequence?

Say I want to have a sequence of formulae H6-A1, H7-A1, H8-A1…

How can I keep LibreOffice from increasing the row part of the second variable?

Because what I end up now when I drag the small square of the selection is H6-A1, H7-A2, H8-A3…

You only need to put a $ to convert this part of the address in absolute, keeping their value when copying or drag.

Please take a look in the help or in documentation to learn more about absolute/relative addresses.
Chapter 7 Formulas and Functions.