How to set the ActionMailer default_url_options's :host dynamically to the request's hostname?

I am trying to set the :host for action mailer default url options.

I have the below set in all the environment files

config.action_mailer.default_url_options = {
  :host => "localhost"
}

I want to make it more dynamic by providing the request host.

when I try to set it by

config.action_mailer.default_url_options = {
  :host => request.domain
}

OR

config.action_mailer.default_url_options = {
  :host => request.env["SERVER_NAME"]
}

It throws error... doesn't recognize "request" object

is there any way I can set this to the request host, not by hardcoding...?