I am assuming that you have this relationship:
Members::MemberID = Addresses::MemberID
If you have a different relationship--such as using a join table so that many members can link to the same address record, let me know.
Define a serial number field to uniquely identify each address record: AddressID. For existing records in this table, you will need to use Replace Field Contents to give them serial numbers.
Define a field in your Members table as: PreferredAddressID.
Use layout tools to draw a square in the row of your portal.
Use the layout text tool to add the letter x, sized and formatted to fit the center of the square.
Use this conditional format expression:
Members::PreferredAddressID ≠ Addresses::AddressID
To specify a font size of 500 to make the X disappear for all but the address selected as a preferred address.
Write this script:
Set Field [Members::PreferredAddressID ; addresses::AddressID ]
(I don't recall if its needed, but you may need to add a second step: Refresh window  after the set field step.)
Now use button set up to turn the Letter X into a button that perfroms the above script.
When you click the center of the square, the script will update the PreferredAddressID field and the conditional format on the letter will update to display the X to show a selected check box. (You can group the box and Letter X before using button setup if you want to turn both into a button or you can use other symbol text--such as a tick mark in place of the box and x set up described here.)
You can also create a new occurrence of your address table and link it to your Members table with this relationship:
Members::PreferredAddressID = PreferredAddress::AddressID
If you find you need that relationship in your database system.
Perfect! Works great! Thanks.
Besides field names, of course, I changed the font color instead of size to make it visible/invisible. Still works great.
I recommend changing the font size. With that set up, you don't have to update the conditional format expression should you decide to change the layout background color at some point in the future.
I understand. Thanks again!