If the records you want to see in the portal will all have the value 'Yes' in the text field, then filter the portal to be:
MyTextField = "Yes"
Say you have field in the portal's table called "Fruit" and you want the portal to display only records where the value "Peach" is entered in this field:
PortalTableName::Fruit = "Peach"
Is all you need.
You can also use:
PortaltableName::Fruit = GlobalFields::SelectedFruit
and then you can put the SelectedFruit field on your layout with a drop down list of fruit names to make the portal on where the user can select different fruit names and see the records in the portal change to display the specified records.
Refresh Window [flush cached join results]
Is needed to update the portal each time you select a value in the global field if you use that option.
If the field in question is a check box field, a different expression may be needed to check for a specific value in the list of selected values stored in that field.