I am totally disabled. How do I enter the form where a doctor signed ?

You don’t need to file it. Just keep it in the case of an audit. 

There is no option in TurboTax to select that you are disable or to enter the form. 

However, Disability tax credits are available to US citizens and residents who are either 65 or older or permanently and totally disabled, as certified by a physician.

are you completing Schedule R? if so here are the rules for the physician statement. it might not have to be filed with your return. if filing is required, you'll have to mail in your return because there is no way to attach this statement to an e-filed return.

Part II — Statement of Permanent and Total Disability - Schedule R
If you checked box 2, 4, 5, 6, or 9 in Part I and you didn't file a physician's statement for 1983 or an earlier year, or you filed or got a statement for tax years after 1983 and your physician signed on line A of the statement, you must have your physician complete a statement certifying that:
• You were permanently and totally disabled on the date you retired; or
• If you retired before 1977, you were permanently and totally disabled on January 1, 1976, or January 1, 1977.
You don't have to file this statement with your tax return. But you must keep it for your records. You can use
the Physician's Statement later in these instructions for this purpose. Your physician should show on the    statement if the disability has lasted or can be expected to last continuously for at least a year, or if there is no reasonable probability that the disabled condition will ever improve.

If you file a joint return and you checked box 5 in Part I, you and your spouse must each get a statement

https://www.irs.gov/pub/irs-pdf/i1040sr.pdf