>I want to sum textbox1.value + testbox2.value
>and store this values in text3, as the user type values
>
>All textboxes are has values from tables, not variables
>
>I put this in interactivechange and kepress methods but could not see
>result in field3. Looks like operation lock the textbox
>
>store field1 + field2 to field3
>Thisform.refresh

In the sample code you provided, you omitted the .Value part.

You can store directly to the textbox:

ThisForm.TxtField3.Value = ThisForm.TxtField1.Value + ThisForm.TxtField2.Value

Refresh is not required in this case.

If you want to save to the underlying field, STORE won't work - that will create a variable with same name instead. Use the REPLACE command instead:

replace Field3 with Field1 + Field2
ThisForm.Refresh()
* or refresh only the TextBox:
* ThisForm.TxtField3.Refresh()

I am assuming the fields are called Field1, Field2, and Field3, and I am naming the textboxes after the fields (Txt + fieldname).