func (c *Client) SubmitForm(ctx context.Context ) (string, error) {
     formVals := url.Values{}
     // Payload created here
     apiUrl := url.URL{Scheme: "http", Host: "api.mytestservice.com, Path: "/submit"}
     httpReq, err := http.NewRequestWithContext(ctx, http.MethodPost, apiUrl.String(), strings.NewReader(formVals.Encode()))
     httpReq.Header.Set("Authorization", <sometoken>)
     httpReq.Header.Set("Content-Type", "application/x-www-form-urlencoded")
     resp, err := c.httpClient.Do(ctx, submitPickupSchedule, httpReq) // This calls to a mock httpClient.Do()
     // error handling and return values goes here
     return resp, err
}

mockHTTPClient := mock_httputils.NewMockHTTPClient(ctrl)
client = Client{httpClient: mockHTTPClient} // Using the mock HTTP client here
t.Run("should call the Do with post request successfully", func(t *testing.T) {
	ctx := context.Background()
	ctx = context.WithValue(ctx, utils.CTXAuthTokenKey, "value")
	mockHTTPClient.EXPECT().Do(ctx, "SubmitCall",
		gomock.Any()).Return(&http.Response{StatusCode: http.StatusOK, Body: io.NopCloser(bytes.NewReader([]byte("SUCCESS")))}, nil)
	resp, err := client.SubmitForm(ctx)
    // assertions here and everything works as expected. It calls the mock method.
}

 func TestArgumentCaptor(t *testing.T){
		var req *http.Request // To record the request attributes
        // Original HTTPClient Do(ctx context.Context, name string, req *http.Request) (resp *http.Response, err error) has this signature
		mockHTTPClient.EXPECT().Do(ctx, "Submit", gomock.Any()).DoAndReturn(
			// signature of the anonymous function must have the same method signature as the original method
			func(argCtx context.Context, argName string, argReq *http.Request) (resp *http.Response, err error) {
				req = argReq
				return &http.Response{StatusCode: http.StatusOK, Body: io.NopCloser(bytes.NewReader([]byte("SUCCESS")))}, nil
			})
        mockHTTPClient.DoCall() // Calls the mockHTTPClient.Do() method.
		// URI check
		assert.NotNil(t, req)
		assert.Equal(t, req.URL.Scheme, <expected-scheme>, "URL scheme mismatch")
		assert.Equal(t, req.URL.Host, <expected-hist>, "Host mismatch")
		assert.Equal(t, req.URL.Path, <expected-path>, "Path mismatch")
		// Headers check
		assert.Equal(t, req.Header.Get("Authorization"),<expected-header>)
		assert.Equal(t, req.Header.Get("Content-Type"), <expected-header>)
 }