Adding “return” leg to companion voucher booking

[M]

Hi, this morning (thanks to the article!) I booked a one way flight from KUL to LHR using a companion voucher.

I believe that I could add a “return” leg to the journey to another Asian city, months later.

What are the exact rules regarding this?

I believe I can fly to anywhere in Asia (it just needs to be closer to KL than to London).
When can I book this? I presume any time before I fly the first “leg”?
And how far in the future can the “return” leg be? Is it one year from the first flight, or is it bound by my voucher expiry date?
And how do I do this exactly? Presumably I can call?

Thanks!

[NorthernLass]

Reading the articles and threads here will be a big help, especially this one:

https://www.headforpoints.com/forums/topic/booking-return-leg-with-companion-voucher/