Open your phone's Contacts app.
Create a new contact.
Enter their phone number in the international format:
Start with a plus sign: +
Then enter the Indonesian country code: 62
Followed by the Indonesian mobile number, without the leading '0'.
For example, if the Indonesian mobile number is 0812 3456 7890, you would enter it as +
Save the contact.
Open WhatsApp.
Go to your Chats screen and tap the new switzerland whatsapp number data chat icon (usually a speech bubble with a plus sign).
Your contact list will appear. You should now see the Indonesian contact you just added. If not, you might need to pull down to refresh your contact list in WhatsApp.
That's it! You should now be able to chat with the Indonesian number on WhatsApp.
Let me know if you have any issues or further questions.