Check out the documentation in my link, above. It includes examples.
Below is the code I wrote after reading those examples. First I set the options into a variable fadeOptions . Then I used that variable to pass the options into the effect.
I set the duration to one second and the delay to zero. It worked. I played around setting to different delays and durations, and it all worked well.
let fadeOptions = {
"duration": 1000,
"delay": 0
};
export function button11980_mouseIn(event) {
$w("#box1980").show("fade", fadeOptions);
}