First create a TextField that listen to the Enter and react like a Tab key [JDK11]

import java.awt.*;
import java.awt.event.*;

public class MyTextField extends TextField {
    MyTextField(int len) {
        super(len);
        addKeyListener(new KeyAdapter() {
            public void keyPressed(KeyEvent evt) {
                int key = evt.getKeyCode();
                if (key == KeyEvent.VK_ENTER)
                    transferFocus();}});
    }

to use it, try something like this:

 import java.applet.*;
import java.awt.*;
import java.awt.event.*;

public class TestEnterAsTab extends Applet {
    MyTextField t1, t2;
    public void init(){
        MyTextField t1 = new MyTextField(10);
        MyTextField t2 = new MyTextField(10);
        add(t1);add(t2);
    }
}