In order to achieve that, you would need to:
- Create a single relation between Table A and your User Profiles table. An easy way to do that is:
- create a template column in Table A and fill it with the email address (or RowID) of the currently signed in user
- join that column to the corresponding column in your User Profiles table to form the relation
- Use a Lookup column to retrieve the current value of FX, via that relation
- Use a Math column to add 1 to that column
Then when your user taps the button on A, your custom action can do a Set Columns on FX in your User Profiles sheet, via that single relation, and using the value of the math column to set the incremented value
This workaround is required because it is not possible to directly apply an Increment via a Single Relation.