Should be so easy, but I forgot how to do it...ggggrrrr....!
 

Alright, I am trying to add a multiple-line box as a custom user profile field. I cannot remember how to do it. I have already made the field, I just do not remember the code to make it show up on the forums itself. Thanks.

Are you serious...nobody can answer this? I know it is not that hard.

When you make a custom user field using the adminCP, on the bottom you can choose where to show it:
If you want it to show in every post, you'll have to edit the postbit or postbit_legacy template.