Subject
for a mail item is a property. This means that you can (typically) query it or assign a
single value to it. So if you want to assign multiple values which originate from different text boxes, you first need to concatenate them, one way or another, and then do the assignment.
For example, you can try the following
mailItem.Subject = $"{txt_CHKR.Text} {txt_PN.Text}";
Few links you may find useful:
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Properties - C# Programming Guide | Microsoft Docs[
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$ - string interpolation - C# reference | Microsoft Docs[
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