One thing you may try (I haven't):

Dim x AS TextBox
Set x = txtHeader(0)
CallByName frmMain,"x",vbLet, "Testing1"

>CallByName cannot be used with control arrays.
>
>>Hi,
>>
>>One of my form which is using the CallByName function is giving
>>run-time error of '438', does not support this method or properties.
>>The frmMain is one of my form name, while txtheader is actually
>>an array of my textbox which start from 0 to 5. I want to be
>>able to control which of the textbox to output my display.
>>Please kindly advise me on to solve this issue. Thank you
>>
>>
>>My code below all give the same error.
>>
>>CallByName frmMain,"txtHeader(0)",vbLet, "Testing1" <<=== not working
>>
>>or
>>
>>CallByName frmMain,"txtHeader0",vbLet, "Testing2" <<=== not working
>>
>>
>>But
>>
>>CallByName frmMain,"txtHeader",vbLet, "Testing" <<=== work