It's not possible, sorry.
In the interest of a canonical answer: The MDN documentation isn't explicit about this, but you can read the proposal's README in GitHub for more information. It says:
The following is not supported, although it has some use cases; see Issue #18 for discussion:
- optional property assignment:
a?.b = c
In the linked issue are the comments 1:
OK, seems like there's a rough agreement in this thread not to do the write case in the first iteration.
We also discussed this question at TC39, and the committee did not seem so interested in adding this feature.
So I guess it's not likely to happen anytime soon.
Hope that helps; good luck!