From the help page on bounty:
How do I start a bounty? When can I start a bounty?
A bounty can be started on a question two days after the question was
To start a bounty, click on the "start a bounty" link at the bottom of
an eligible question and allocate anywhere between 50 and 500
reputation, in 50-point increments. You can also choose from a list of
bounty reasons to help other users understand why you’re dissatisfied
with any current answers to the question. If you need to describe
something in more detail, there’s also optional, additional custom
text that will be displayed with the bounty reason.
If you actually tried starting a bounty, you would get a dialog for entering your custom reason, which is the third and final step when starting a bounty.