I'm translation a website for someone, and I come across the following:

A code has been sent to your email address %s. _____ the code here to unsubscribe:

Which is more common to fill in the gap: 'fill in' or 'enter'

The idea is that visitors have to type a code in a text field to unsubscribe.

Back then when I used to do some software development I would usually word it 'key in' or 'type in'. In my opinion it makes it clearer that the user is expected to enter something by means of the keyboard.

Although 'Fill in' is good for web-oriented applications when they aim at implementing a notion of 'electronic form' ; for instance some kind of 'subscribing' document or application form. But then the idea would be that the user has to fill in the form in general ; so when it comes to a given field within the form I'd rather be more specific as to which action is expected. In which case 'key in' states that the value has to be entered on the keyboard as opposed to picking it from a drop-down list for instance.

But all this is just me talking... Good luck with your site development work anyway !
Enter is the usual word for this situation.