You can’t. It is not an official alias. That is the point. So they will be seen as separate accounts. But I forward everything to he same account. Have to configure it for every email address. This way I keep a whitelist.

Don’t use + as delimiter. It is an email standard for alias and will send the email to the none alias version inbox. Better to use a different delimiter, like minus, -, instead. Not sure how easy it is to do with Proton.

I keep a whitelist of email addresses allowed, the rest is therefore blocked. This means I can’t send to random@example.com and get to the inbox.