Monday Muddle: opt in, opt-in

Monday Muddle: opt in: (phrasal verb) choose to enroll; choose to register; choose to participate Used most frequently to indicate signing up online for an email list. opt-in: (adjective) usually used before "form" or "box" to describe the place where someone can choose to register; (noun) a shorter version of "opt-in form" For example: My opt-in is on every page of my website.

I have seen “optin” as one word without a hyphen used as a noun occasionally. It is not yet standard, but I expect it will be one day. If you choose to use this variant, be aware that autocorrect may change it to “option”.