just to add to this... Whenever I try to create a new WebAppFunction I'm getting this.

azure-native:web:WebAppFunction (post-to-slack):
    error: Code="BadRequest" Message="Encountered an error (InternalServerError) from host runtime." Details=[{"Message":"Encountered an error (InternalServerError) from host runtime."},{"Code":"BadRequest"},{"ErrorEntity":{"Code":"
BadRequest","Message":"Encountered an error (InternalServerError) from host runtime."}}]

I'm trying this out (the ScriptRootPathHref is optional, gives me the same error without it)

var waf = new WebAppFunction("post-to-slack", new WebAppFunctionArgs
            {
                ResourceGroupName = rg.Name,
                Name = app.Name,
                FunctionName = "post-to-slack",
                Language = "node",
                FunctionAppId = "post-to-slack",
                ScriptRootPathHref = codeBlobUrl
            });

Not sure how to proceed, any help would be appreciated