You may have seen blogs that have a link to their askbox on their page that does not go to their URL and "/ask" added to the end - the page with simply the ask title and then the actual box. It goes to a different URL and contains an FAQ or an "about" page, and under this is the askbox form. (EXAMPLE)
This is actually a very simple thing to do.
First, you go to the customize page - either the classic or the new one. You go to pages and create your new page with the URL that you would like the message form to be at. I usually use "message" or "mail" for mine, just to make things easier on myself.
After you've created the page, go to the button that says "HTML" above where the text is to be inputted. This will allow you to put in a code.
Next, go to this page and copy the code. Paste this code into the HTML box.
THE MOST IMPORTANT PART: Make sure that before you save the page, you change the part of the code that says "YOURURL" to the URL of the page that it is on. Do not include the ".tumblr.com" or "http://" in this part or it will not work.
Now that you've done this, click update at the bottom of the HTML form. The ask box should be there when you go back to the page form, and you can add your title and any other text then.
Don't forget to save the page!