How we can assign account definition to the users while creation

One way is to run a process on an "INSERT" even on the person table which inserts an entry into PersonHasTSBAccountDef table - ofcourse this is quite a simplistic view on things but it will assign the account definition you need to the person object automatically

Assignment of a dynamic role could work here: the newly created Employee is assigned an account definition dynamically via inheritance, i.e a Business Role.

You need to ensure the criteria of the dynamic role allows for immediate assignment.  But this is pretty typical of a configuration where creation of accounts is required.

Trevor