Some Basic Questions Regarding Forms

Hello, very basic user here.

I've created a form that has a text field that concatenates values using:
=![PREVOWN] & ![PARKNAME] & ![TRNSNUM]
I know concatenation is a big no-no with normal forms, however, the agency I work for, wants to use this concatenated value as their unique identifier. Using a query alone is not feasible as the database was set up to use this value as the primary key, so we need to include the value.

I am having two problems:

The first is that the field [TRNSNUM], is an incremental autonumber field. The values in this field need to start at 10,000 so I used a Format of "10"000.
This works great in the [TRNSNUM] field itself, but in the concatenated field, I am not seeing the first two digits, so my values for the last third of the concatenation are showing up 123, 124, ... instead of 10123, 10124. Any help on this issue? Also, is this just assigning the two digit field of 10 before my numbers? If so, I need a better way to do this, as my values will reach up to 11,000 and so on.

The next issue, and the larger problem is that my concatenated text field is not being input when I click on my forms, "submit" control button. I used the command button wizard to create this button. I selected "Record Operations" and then "Save record." It seems to save all of my other text field, except the concatenated field. This is a problem, since the value is supposed to be my primary key, so if I cannot obtain the value from my form, I cannot add more records, as the primary key cannot be null.

Any ideas would be of great help, I am a very beginning user, with very little knowledge of SQL or VB, so I am slogging my way through.
Thanks.