How to use a custom button to insert into a field on the same page

What is the best way to use a Custom Button to add a preset text to a field on a current form?

Lets say I have a field [Status] and I would like to have two buttons "ACTIVE" and "COMPLETED". When you press one of the buttons that preset text would be added to the [Status] field and then the user could save the record.

While a drop down box could work, I am looking for something just a bit quicker to change the status.