testscript.js

//To test, just add the script and the sheet to any blank game, then create a new blank Character in the game, then do any chat message.
//Expected behavior:
// "Found webworker script"
// "API Ready."
// "Hello from sheetworker"
// "Creating baseattribute..."
// "Base attribute heard change"
// "Created attribute heard change"

on("chat:message", function() {
    //Find the first character and just use it
   var firstChar = findObjs({type: "character"})[0];
   if(!firstChar) {
       log("You must make a character manually");
       return;
   }
   log("Creating baseattribute...");
   var newattrib = createObj("attribute", {characterid: firstChar.id, name: "baseattribute"});
   newattrib.setWithWorker({
       current: "1"
   });
});

on("ready", function() {
    log("API Ready.");
})

testsheet.html

<input type="text" name="attr_baseattribute" />
<input type="text" name="attr_createdattribute" />

<script type="text/worker">

    log("Hello from sheetworker");
    
    on("change:baseattribute", function() {
        log("Base attribute heard change");
        setAttrs({
            "createdattribute": "12"
        });
    });
    
    on("change:createdattribute", function() {
        log("Created attribute heard change");
    });
    
</script>