Run a Javascript function from an Outlook Add-in Manifest file

I’ve setup an Outlook Add-in using an XML manifest file. I’ve got some buttons appearing in a menu drop down on the UI. When I click the buttons it says my request is being worked on and does nothing (I’m expecting message boxes on the click of these) <attached>

I’ve tried checking all the references and files involved. Double-checked the xml for the button, function is ShowAlert1:

<DesktopFormFactor>
            <FunctionFile resid="functionFile" />

            <!-- Message compose form -->
            <ExtensionPoint xsi:type="MessageComposeCommandSurface">
              <OfficeTab id="TabDefault">
                <Group id="msgComposeDemoGroup">
                  <Label resid="groupLabel" />
                  <!-- Function (UI-less) button -->
                <Control xsi:type="Button" id="msgComposeFunctionButton1">
                  <Label resid="funcComposeButtonLabel" />
                  <Supertip>
                    <Title resid="funcComposeSuperTipTitle" />
                    <Description resid="funcComposeSuperTipDescription" />
                  </Supertip>
                  <Icon>
                    <bt:Image size="16" resid="blue-icon-16" />
                    <bt:Image size="32" resid="blue-icon-32" />
                    <bt:Image size="80" resid="blue-icon-80" />
                  </Icon>
                  <Action xsi:type="ExecuteFunction">
                    <FunctionName>showAlert1</FunctionName>
                  </Action>
                </Control>

My function file is:

<bt:Url id="functionFile" DefaultValue="https://localhost:3000/Functions.html"/>

Within Functions.html is the ref to the js file with my function:

<script src="Functions.js" type="text/javascript"></script>

and finally the Functions.js has my function def:

function showAlert1() {
    alert("This is an alert box!");
    
    event.completed();
}

ALL three of these files, the manifest, the functions.html and functions.js are in the same folder. I’m running a localhost and it hasn’t complained in red about any of these files….
Can someone help to debug this or am I doing something obviously wrong?