Changing the Eclipse Content Assist Decision Keybind

@ Since there is no OS or keybind setting (Windows or Emacs) that you are using, I will answer for Windows once.
You need to change two places from the default keybind.

Open Window=>Preferences from the Eclipse menu.
Select General=>Keys from the side menu to open the keybind menu.
In the text box where you enter Filter, type Next Column and set the Enter key to the Next Column that appears there.

Now, when a candidate comes up, you can use the Enter key to get through the candidate.
If this is the only way to do this, the new line will disappear and behave strangely when the new line is broken, so I will set the following additional settings:

Type Insert Line Below in the text box where you enter Filter and set Enter to the Insert Line Below Current Line that appears.

This allows you to ignore candidates and insert line breaks as before.
However, please be careful because you will register the Enter key that you want to use as a standard, so there may be unexpected disadvantages of the operation that has been taken for granted.