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You will need to build your own web data connector as per the developer docs and examples here - Web Data Connector
When you say you're using simulator - you cannot just put in the URL of your existing service. You actually need to input the URL to your Tableau Web Data Connector you've built (or will build).
So effectively you're going to end up with a Tableau Web Data Connector (which an HTML page) which connects to your Web Service to retrieve the data and your WDC will format/transform and return your data to Tableau.