Since the local ASP.Net site does not have knowledge of the external site, just enter your code in plain HTML:
<a href="http://external......">....</a>
If you need to calculate the parameters:
<a href="http://external......?<%= MyParamsCorrectlyEncoded() %>">....</a>
If you use MVC (your question mentions Url.Action), you can use AttributeEncode to encode attributes to an external site.
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