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.
and 2:
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!
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