adding markdown to a textarea

how can I use the hook for before submitting a form, to get each textarea individually and apply a function to it?
ive been working with this:
i need some sort of for each loop to grab each one and then apply the markdown before submission.
also... is there a way to add a media button to a textarea?

add_action("gform_pre_submission", "pre_submission_handler");

function pre_submission_handler($form){
//use markdown
$_POST["input_14"] = Markdown($my_text);

}