I’m pretty sure this is not possible with current vanilla Discourse.
So the question back to the OP is: please describe how you would like this to work?
So you give them:
- https://mysuperamazing.com/signup or https://mysuperamazing.com/sup3rs3cr3tK3y and anyone can forward this to anyone and they can use the link - what have you gained from the vanilla URL?
If you just use the vanilla URL and use any suitable mail service to send out the mass email and you’ve achieved what you need to achieve?
Using your Discourse to send out this spam is going to risk your sites reputation and all email delivery reliability going forward.