How to pass a variable for key/value pairs in an argument?

Many functions use Arrays as arguments so your question is really how do you construct an Array?

If you have a variable as a key then it’s better to set it using square bracket syntax.

// key pairs in array constructor

$args = array(
    'key' => 'value',
);

// key pairs outside of array constructor

$key = "This is my key";
$value = "This is my value";

$args [ $key ] = $value;

// pass your array now

do_action ( 'xyz', $args );