They work by giving you a random email address that is available for a short time (ex. 10min) and an inbox for any messages you recieve during that time (ex. an email verification message from a Lemmy server)
I didn’t want to recommend a specific one, however Googling “temporary email” should give you several options to choose from.
Also bugmenot.com is pretty handy sometimes.
Sadly, a lot of websites are blocking temporary email domains :(
I used jetable.org for years but now it’s fairly commonly blocked in email forms.