Create a field unique in table CustTab

I work on a project around dynamics ax2012 , and i want to add some fields in CustTable .

I want to add a unique field  that has 11 caracter consists of 4 digite 

1er : 1 caracter for sexe

2:     2 caracter for year of birth

3 :    2 caracter month of birth

4 :    6 caracter  this caracter should be auto- increment

How can i produce to do that , and when i create new form how  can i  implement this field automatically?

Any idea

Thank you