MS Access - Code for a button

Just starting in Access ... I need help with the following.

I have BRAND_NAME in the current navigation form ( refernences a field of the same name).
I have a button on the same form... I want when the button clicked to check on a table called BRAND_NAME has one field CALLED BRAND_NAME. If that value does not exist, then add the value. If the value exist then ignore. I also need that comparing the value doesn't care about case.

How can I do that?

I am started over 40 Years no school... Just for a hobby / side work.


Thanks
Emad