Test cases:

f($var) = awk -v var="${var}"  \
     'BEGIN { OFS=FS="="}
     { gsub(/_/,"-",$1); if ( $1=="--"var ) lastval=substr($0,length($1)+2) }
     END { print lastval }'

Missing input is blank:

$ echo '--var_aa=something' | f(var-b-not-ther)
(blank as expected)

All RHS of = is unmunged:

$ echo '--var_aa=password==_-$' | f(var-aa)
password==_-$

Mixed - and _ in var name:
$ echo '--var_aa-bb_cc=1' | f(var-aa-bb-cc)
1

No value returns blank line:
$ echo '--var_aa-bb_cc' | f(var-aa-bb-cc)
(blank line as expected)

Multiples return the last:
$  echo -e "--bb=cc\n--bb=dd" | f(bb)
dd