If your implementation does not accepts HttpClient in constructor and creates HttpClient directly, then you may use IHttpClientFactory.CreateClient() in your code. Now mock this IHttpClientFactory.CreateClient() to return your HttpClient in test.
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This was really helpful. Thanks Lars!
Just want to add:-
If your implementation does not accepts HttpClient in constructor and creates HttpClient directly, then you may use IHttpClientFactory.CreateClient() in your code. Now mock this IHttpClientFactory.CreateClient() to return your HttpClient in test.