日本での就職活動でTOEICのハイスコアが必要って英語でなんて言うの？

The phrase "in order to" tells your listener that first you will state your goal and then you will state what you need to complete that goal. In this case, your goal is to get a good job and you need to get a high TOEIC score.

I hope this helps :)

Both of these sentences above explain that said person needs a high TOEIC score so they can get a good job in Japan.

Decent job' is a more casual way of saying 'good job'. Saying something is 'a must' implies that it is essential and you cannot do without it.
Both of these sentences explain that you need a high TOEIC score to get that good job in Japan.

When talking about getting a good job then another way to say this is to get a 'decent job' it means the same thing
Both of these explain why you need a high TOEIC score in Japan

If someone asks you why you are studying English, and you wanted to explain that you need a high TOEIC score for job hunting in Japan, you could say any of the following: "I need a high TOEIC score so I can get a good job", "I would really like to get a high TOEIC score, so it can help me get a good job" or "If I get a good TOEIC score, it will help me get a good job".

When you want to explain that you are studying English because you need a good/high TOEIC score when you are job hunting; then you can explain this in the following way:
-I need a high TOEIC score so I can get a good job
-I am learning English because I want a good TOEIC score that will be an advantage when I am job hunting