Update individual cell on excel based upon userform textbox dynamically

Hi all,

I would like to update a cell in excel based upon the value of a textbox. I know how to do this bit, but what I want to do is change which cell the textbox updates based upon the value of another text box. I hope that makes sense. See below the VBA code I am attempting to use:

Worksheets("Create Interim Report").Range Reg4.Value = Reg3.Value

As per above, I will have the value of the excel cell I want to update in Reg4 (my textbox - which for example it will say 'D45') and I want that excel cell updated based on Reg3 value. It will work fine if I just say

.Range ("D45").value

but I would like to make it dynamic.

Any help would be appreciated.

Thanks all in advance.