@bmarcowka You can set a variable OnSelect of the icon and make the default value of TextInput is equal to that variable. You need to set the variable before submit form operation. So you code look something like below.
Remember to TextInput Default is equal to textvalue.
Correct me if I am wrong but shouldn't it read?
With MANUPDATE the field I am looking to update.
Here you are assigning MANUPDATE variable is equal to "Y". And make the deault value of the TextInput is eqaul to MANUPDATE. So when we click the button the default value will become "Y". Another way is you can use PATCH function to update the item in SharePoint list or any other datat source.