The answer to this depends on what you mean by “absorb.” There will be a different answer if you are talking about Germany taking a country over without fighting than there will be if you are talking about military conquest and absorption.
If you are talking about military conquest, the correct answer would be Poland. Germany invaded Poland in September of 1939. It defeated Poland in about six weeks. It divided up conquered Poland with the Soviet Union as it had agreed to do before the war started.
However, I think that the better answer is Austria. This was the first country that Germany absorbed before the war started. It did so in a semi-voluntary annexation that is known as the “Anschluss.” Much of the population of Austria welcomed this union with Germany because the people of both countries were ethnically German. Even so, Austria ceased to exist as a country for that time. Therefore, it is proper to say that Germany absorbed Austria.