Force user to enter specific alphabets in userform

hi excel experts
I have a userform where a textbox is used to input the aplphabetic code name of a Raw material. The first two alphabets to be used in the textbox are 'R' & 'X' respectively. After typing in 'RX' the user can type in any other alphabets. My problem is how to force the user to type R & X first before entering other alphabets? Any clue friends...... Thanking you in advance

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Saji,

Just to be clear, JasperD's code would at "RX" to the beginning of whatever your user inputs into textbox1 IF they haven't entered it themself.

Thank you JasperD. That solved my problem