In order to expose your custom control's text boxes to the shared spell checker on the form tab, you need to override the
GetSpellCheckFields() method in your control. It needs to return a
List<string> with each of the text box controls client IDs. This can be retrieved with the
ClientID property of your control.
Here's an example:
public override List<string> GetSpellCheckFields()
List<string> result = new List<string>();
// txtTest is the ID of the control you want to spell check.