Thank you for reaching out. If I understand it correctly you are unable to save the new PSK via Azure Portal as the Save button is greyed out.
I found a similar issue internally and got to know that this is a known issue, and the team is working on fixing it.
The work-around suggested in this case is to access the Azure Portal via https://aka.ms/vpnportalflight which has the fix.
Hope this helps! Please let me know if the issue still exists.
Thank you!
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