How to attach a statement? Use a stapler?  If filing electronically, there is a place on the ProSeries Schedule A page to enter that information.  Otherwise, read the instructions:

Interest reported on someone else’s Form 1098. If you and
at least one other person (other than your spouse if filing jointly)
were liable for and paid interest on the mortgage, and the home
mortgage interest paid was reported on the other person’s Form
1098, identify the name and address of the person or persons
who received a Form 1098 reporting the interest you paid. If you
are filing a paper return, identify the person by attaching a
statement to your paper return and printing “See attached” to the
right of line 8b.