Craigslist help on ipad

Hi
Can anyone Please tell me how to do this from your iPad ? I am trying reply to Someone post using webmail. I am using my yahoo email. and can not Figure out how to do this.

I am trying to reply to someone Within Craigslist I click on reply on the post it Brings up a window with reply using webmail. I click on yahoo email. In my yahoo email a Window pops up new email. But can not find away to put That person email Address in to the box where is Says to . Thank you. for any help on this