Message transmission method in the dialog in the dialog box?

[BK]

go_site:
PostMessage tab_test_dialog_handle WM_APP 0 0

tab_test:
,case WM_APP
,out "message received"

Function go_site

Code:

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;messages2
sel wParam
,case 3
,PostMessage win("tab_dialog" "#32770") WM_APP wParam lParam
,case IDOK
,case IDCANCEL
ret 1

Function tab_test

Code:

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,case WM_NOTIFY goto messages3
,case WM_APP
,,out "message received"

I'm sorry but there is a part that does not make sense.
If there are Google and Yahoo button two or more buttons
How do you recognize two distinct buttons?
You can give an example to illustrate how to use it?