I have a main form. And I have a text field named text123. when the user clicks the text123, it opens another form. In this new form there is two controls. one is a combo box named cmb1 and another is a textfield named text456. when the user selects an option from the combo box the lookup value is placed in the textfield on the form.

我有一个主表格。我有一个名为Text123的文本字段。当用户单击文本123时，它会打开另一个表单。在这个新表单中有两个控件。一个是名为cmb1的组合框，另一个是名为ext456的文本字段。当用户从组合框中选择一个选项时，查找值会放置在表单的文本字段中。

now I have an appen button on this form. How do I pass the value of the text456 in the second form to the text123 on the previous form when I click on the append buton.

现在，我在此表单上有了一个Appen按钮。当我单击Append Buton时，如何将第二个表单中的文本456的值传递给前一个表单上的文本123。

an example of the second form is shown below:

第二种形式的示例如下所示：

The easiest way to get the correct reference is to open both forms.
Then go select the form from which you want to update the other form.
Select a field and open in Properties Control Source and click on the ...

获取正确引用的最简单方法是同时打开两个表单。然后选择要从中更新其他表单的表单。选择一个字段并在属性控件源中打开，然后单击...

Then you will see the following Expression Builder window. In that window click on the + in front of Loaded Forms. Then you can select any control from any open form.
This gives you the exact syntax you need.

然后，您将看到下面的表达式生成器窗口。在该窗口中，单击加载的表单前面的+。然后，您可以从任何打开的表单中选择任何控件。这为您提供了所需的确切语法。

Just change
Forms!MainForm.text123.Value = Me.text456.Value
to
Forms!MainForm**!**text123.Value = Me.text456.Value

只需将Forms！MainForm.ext123.Value=Me.ext456.Value更改为Forms！MainForm**！**ext123.Value=Me.ext456.Value