Just signed up for a Proton mail account and very disappointed not to have this feature.
This has been reported so often that Im surprised that Proton refuses to offer it.
I will continue to search for another alternative to Gmail that does. Good thing I did not already choose the paid Proton service.

The fact that this is not implemented is shameful.

Special handling for Reply-To is not critical, but being able to add standard email headers definitely is. There's no good reason not to have an escape hatch.

Here too.

Ping! Still mission critical for me.

Absolutely essential!

How is this still not a thing?

Must have when your main domain is hosted elsewhere (and has to remain elsewhere). Mail forwarding and reply-to are jointly the solution to my situation.

This is a must-have for me and is supported on practically all other platforms. Have been waiting for a comment from Proton for a year now. I'm going to move away from paid version to some other platform at the end of this billing cycle.

The reply-to field allows the sender to specify a preferred email address for reply mail. This feature is present in most modern email services and is a standard feature of email.

Others suggest the posibility to change the from field to an +alias.
https://protonmail.uservoice.com/forums/945460-general-ideas/suggestions/45639766-alias-creation-when-composing-an-email
Without any of these suggestions I do not see the purpose with +alias.
+alias can also help feighting Spam as you can change your public mail alias without changing your mail account

I need a reply to to work with my helpdesk software. Without it I'll have to find email alternatives

I like to use different email reply addresses for different companies *, and this would facilitate that.

* because companies I deal with keep getting their data stolen (3 times this year already) along with my details, and I can then selectively block the inevitable spam messages.

As an old-time member of ACM.ORG, I have access to a forwarder, and I'd really like to be able to go back to replying to emails sent to my @acm.org address with that as my reply-to address.

This is VITAL! I cannot use Proton Mail for all my email purposes until it has the ability like Mozilla's Thunderbird email client for my email messages and replies that I send to ORIGINATE FROM whatever domain and address I choose (for domains I forward to Proton Mail and have appropriate DNS SFP records set to permit mail origination from Proton's ervers--or for replies to messages I have received in my Proton Mail mailbox because Proton is set to accept email to ANY or ALL addresses for that domain). As it is now, I rely on a different mail server for handling this. It would be nice to move all my email to Proton, but until I can set identities for messages and responses for domains I own, I cannot use Proton Mail like I wish that I could.

I had to cancel my paid account after waiting 3+ years for this. I'm using another service that's not as good, but allows me to reply to threads using my real e-mail address that will remain portable between providers.

Critical for me to move to Protonmail from GMail.

While using e-mail forwarding service and want all replies coming thru this service. Having this in ProtonMail clients would be highly appreciated.

Still no comments from Proton Mail, bumping the thread, this feature is critical for my continued usage of Proton Mail

An official response from Proton Mail (Accept, delayed, reject - with rationale, etc.) would be appreciated.