It might not be clear enough, anonymous_user_03c2c3b0 already answered their own question 3 month later
‘child widget’ → ‘Get slot’ → ‘cast to Button Slot’ → ‘Set Horizontal Alignment’
You can also right click on the cast function and select ‘Convert to pure cast’ if you don’t like the execute pins
This is how all the ‘Slot as Slot’ functions work. I guess someone at Epic Games just forgot to simplify it for the Button slot.