How to lock cell based on first value?

Hello,
I have a multisheet excel workbook that I am programming. I was wondering if there is a way to lock a cell on one sheet after the initial value was put in, allowing me to change it later. This may not be so clear so here is the problem....

I have Workbook1. The input from cell A3 on Sheet1 is 5. I have made a second sheet called Order Form. In cell B5 of Order form I have made it ='Sheet1'!A3. I am looking for a way to be able to change cell A3 on Sheet1, but keep the 5 on Order Form. Is this possible??

It's almost like a one time input.

Any help will be appreciated.

Thanks,

Put this in the worksheet object module for Sheet1:

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When you type something in Sheet1 A3, it will be copied to Order From B5 only when B5 is currently blank.