How can I buy free return tickets for a month-long stay abroad?

I've seen some videos on YouTube, but
I'm posting this to get some advice from the Bulpe community.
I'm planning to live abroad for a month,
but since I don't know exactly when I'll be returning,
I've noticed that immigration usually requires a return ticket.
Last time, I booked a return ticket through Agoda, and after calculating the potential 100,000 KRW loss,
I ended up losing 30,000 KRW.