Add to a multi select on a linked record

I am passing in the id of a linked record from a trigger, and I want to add to a multi select. How can I use push() or get the existing values of the field and add to them? This is as far as I could get.

let inputConfig = input.config();
console.log(`${inputConfig.recordid}`);

let table = base.getTable("Individual Inventory");
let field = table.getField("Tags");

let rec = inputConfig.recordid;
console.log(`${rec}`);

await table.updateRecordAsync(
      rec[0], {
          Tags : 
  });