This plugin will add a URL to the Indigo Web Server that will allow you to specify a variable and it will get the value of that variable and (assuming it's a valid URL) it will redirect to that URL. This allows Refreshing Image URLs to be more dynamic.
To install this plugin drag the variable_url_redirect folder (after you download an unzip it) to the following location if you are running Indigo 2021.+:
/Library/Application Support/Perceptive Automation/Indigo 2021.x/Web Assets/plugins/
or this location if you are running Indigo 7.x:
/Library/Application Support/Perceptive Automation/Indigo 7.x/IndigoWebServer/plugins/
and then restart Indigo.
To use this in a Refreshing Image URL on a control page, you specify the url: http://USERNAME:PASSWORD@localhost:8176/variable_url_redirect/redirect?name=VARIABLENAME
You substitute the appropriate information for the words in ALLCAPS. The username and password are the username and password for your Indigo Server as you specified it in the Start Local Server... dialog. You pass in the variable name (not the id).
Note: this plugin will only work with http/https URLs (not file URLs for instance).
If you changed the port for the Indigo Web Server you'll need to use your custom port number. The above should work for most people though.