Hello,
To achieve something like this:
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You have to provide a special list value on the combo box that you have on the screen.
0 as Value 1 and Option 1 as Please select a color
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To avoid allowing the user to submit an invalid option you may want to use one of the following options(or both):
1. Make sure the mandatory value of the combo box is set to true and if you have a button with the validation type "Client and Server"
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This will make a client side validation and if the value of the variable is zero(variable has to be of type integer/long integer/identifier) it will raise an error near the combo box:
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2. Validate on the server side if the combo box variable is equal to zero. If so, you could raise an error "Please select a valid color"
Kind regards,
Márcio Menezes