Forced Username [$first.$last] dynamically shown on rego page

[GlobalFusion]

Im racking my brains about how to go about this...

On the Registration page (currently using 3.5.3), I have added to extra fields (profile):

- First Name
- Last Name

I have also disabled the username textbox.

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What I want to do, is force their username to be FirstName.LastName, but also have it dynamically show in the disabled 'username' textbox.

I know there are javascript methods to enter into one textbox, and auto enter that input into another, but dont know of anyway to dynamically put the input from both textinputs with the dot in the middle.

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Please... Please... someone tell me :

(a) - It can be done.
(b) - how to do it.


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Thanks in Advance.