I would suggest to split the field into 2 textboxes and combine the values entered afterwards.

>I have set automatic value for a textbox like "RA-'.
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>At the end of this text, the user should be able to add a no., like RA-01..
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>In ADD button click event, I set the value of text to RA and did a setfocus on the textbox.
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>How should I do this to put the cursor at the end of the text RA.. rather than put the cursor before the text.. ?
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>If the text is RA-01.. and so forth.. how can I automatically set the value of the text box to RA-02.. when user clicks the ADD button?
>