You put it on the Steemigram site in the field named "Steemit Title".
You can put a title you choose or you have some "variables" that you can put and that will be automatically converted when posting on Steemit from Instagram.

__STEEMIT__

will be converted to your Steemit username

__INSTAGRAM__

will be converted to your Instagram username

__DATE__

will be converted to the date

__DATETIME__

will be converted to the date and time

And you can also put everything you want in between (and before and after) these variables. just don't forget the caps and the 2 underscores.