I have a Form 8937 with the price of a corporate spinoff supporting the cost basis as of the spinoff date. How do I include it in TT to support the cost basis there?

You can't enter a Form 8937 on Turbo Tax and don't need to - that form is not reported on your tax return.  

Form 8937, Report of Organizational Actions Affecting Basis of Securities is only filed by the issuer of a specified security that takes an organizational action that affects the basis of that security. If you received a Form 8937, you will use the information to adjust the basis of the affected security in order to calculate any gain or loss on the sale of that security.

The adjusted basis would only be reported (on your return) when you sell the security, unless the information on line 18 (Can any resulting loss be recognized) and line 19 (Provide any other information necessary to implement the adjustment, such as the reportable tax year ) indicate otherwise.