I have a choice field (Incentives) that is formatted as a multi select checkbox. If the user includes T-shirt in their one or more selections, then I want to require a selection for T-shirt Size. I am not sure how to word the validation rule on the Size field to accomplish this. The syntax I have works if you have only the T-shirt option selected and I'm not sure how to revise it to say "if T-shirt is included in the one or more choices, then require the Size".
Incorrect/incomplete rule on Size field: {Self} == "" && IncentiveOptions == "T-shirt"