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You need to wrap it all into interpolate:
```pulumi.interpolate`https://${containerAppName}.${managedEnvironment.defaultDomain}````

So, that worked, any idea why this might not work when passing an output from a resource into a module as a parameter and then use that in the module?

as in...

```...
export function scanApp(resourceGroupName: Output&lt;string&gt;,...): {

...
new appservice.Plan(appServicePlanName, {
      resourceGroupName: resourceGroupName,
}```
That doesn't work.
I already tried `pulumi.interpolate`${resourceGroupName}`` but that just throws the same error.

I suspect you call `toString` on an output somewhere but it's hard to diagnose without the full code. Your snippet looks perfectly fine.

I am doing this here later:

```const appName = pulumi.interpolate`argos-${resourceGroupName}`;
    const prefix = `${appName}-${stack}`;```
I then use `prefix` as a property for another resource as well. Does that not work?

<https://pulumi-community.slack.com/archives/C84L4E3N1/p1699997944559479>
Asked, been told :smile:

&gt;  const prefix = `${appName}-${stack}`;
This is your problem. It has to be an interpolate again.

And you won't be able to use it for a plain string value like resource name e.g. `new Resource(prefix + "foo", ...`

yeah, I guess that's fine now. When we started this specific stack over 3 years ago we didn't think too far ahead. It is what it is now :slightly_smiling_face: