Contains a Default Value but need Form Validation

Hello,

I need your help. I am creating a database for work and have run into
a problem...

I am in the process of creating a form.  In one of my fields called
Asset_Num I have put a defalut value of `BC` because I would like this
to appear by default for the user in the field.  I have an input mask
of LL00000 so that BC is already entered and then the user just has to
enter 5 numbers.  I have also put a validation to the effect of: Like
`BC and 5 questions marks to show that it should contain 7 characters.
 This field is a primary field and is required.  Though this
validation does not prompt an error..

The problem is when the form opens up it adds the BC to the field
which I want but then I can tab to the next field without it giving me
an validation error.  I would like it to force me to enter the 5
numbers to the characters already displayed before letting me move to
the next field.

Thank you for your help...