In order to get the ID number for the item in the workflow, you can query ID value from:
You can get the URL from the Site, and the rest from the List.
string itemURL = "http://sharepoint:9999" + this.workflowProperties.ListURL + "/DispForm.aspx?ID=" + this.workflowProperties.Item.ID;
This string will be the URL you are looking for and should come out like:
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