[brongondwana]

You need to OAuth to Fastmail (the service) to hook it up, then as was mentioned above, you can go into the settings in your Fastmail account to choose which domain your Masked Email addresses are created in:

Settings -> Domains -> Team Settings -> Masked email domain

It will default to fastmail.com, but easy to change it.

[capableweb]

Oh, I completely misunderstood then, I thought I could do this with just 1Password. I already have email setup myself and don't need Fastmail, so then it seems I cannot use this feature. I'll just continue myself to randomly generate my addresses then...