set From when replying

I'm sure this has been asked before, and it is even likely that at one
time I knew the answer. But I'm getting old, and the world is
enshittifying at a pace that makes me lose my mind :(

So; I need to set the From when replying to the address which the
original message was To. More precisely, and this is important, I need
to set the *from* *variable*, i.e. a my_hdr command is not enough. My
current configuration already inserts the correct From header, but
leaves the variable empty, with unsatisfactory results (because I use
the variable for another setting later in the config).

-- 
Ian