you need to put some code behind a suitable event (think of an event as a hook to associate a fragment of code which reacts to an action or event triggered elsewhere)
probably the best event is going to be the on change or on lost focus events
you need to use some vba functions to retrieve the first character of each element you want.
id suggest creating a function to do the actual deed
private sub CreateID()
Id = left$(FName,1) & left$(UName,1)
you can then call that function from whatever events you want (id suggest probably the on change and on lost focus events in the code for the controls holding FNAME & UNAME.. in the above example ive assumed that your control names are the same as the column names.
however that skeleton sub routine isn't going to pass muster.. you may want to trap for null values
you may need to inhibit changes once a record has been created.. ie you can only set he vlaue of the id column whne the record is created.. but not after that.
however, before doing that I'd want to check my id is correct and valid.. what meanijn does it have... should Id be unique.. if so then merely taking the first 2 characters will not suffice.. you need to add soem numeric component.
as the two characters are derviced data.. shoudl they even be stored separately...... they after all can be recreated in a query easily enough...
select left$(FName,1) & left$(UName,1) as Id from <mytable>