A HTML/JS request

Hi Guys, I'm new, I'm currently looking for a form witch I require.
I'm looking for a one feild form where I can have he guest put in a code, a Pacific code, I want to have the user proceed to the next page. I'll handle the where the code is coming from, and what of it.

For example, Let's just say I had a form, I put in 1800 into that code, and the user proceeds.

and for a bad example, if I was to put in a code like 3421, It'll show up saying "Please try again, this isn't the right code".

Can anyone help me, I've tried this with my HTML logic, but becouse it's only partly through, I failed. I think my chances are in Javascript. (and I've tried for a 2 hour and more period, including internet searches).

Thanks, and can ANYONE help me?