Form 8283 - Submitting Signature

You are correct that the IRS requires a signature by the charity for donations in excess of $5,000.

Per the instructions: Deduction over $5,000. You must complete Section B of Form 8283 for each item—or group of similar non-cash items— for which you claim a deduction of over $5,000 except as provided in Deductions Over $5,000, later. The organization that received the property must complete and sign Part V of Section B, Form 8283.

You can mail your signed Form 8283 (make sure you keep a copy) to the IRS using Form 8453.  You can find Form 8453 by clicking here.
 

Per the instructions for Form 8453:

• Use Form 8453 to send any required paper forms or supporting documentation listed next to the checkboxes on Form 8453 (don’t send Form(s) W-2, W-2G, or 1099-R). 
• CAUTION Don’t attach any form or document that isn’t shown on Form 8453 next to the checkboxes.
•  If you are required to mail in any documentation not listed on Form 8453, you can’t file the tax return electronically. 
• Note: Don’t mail a copy of an electronically filed Form 1040, 1040-SR, 1040-NR, or 1040-SS to the Internal Revenue Service (IRS). 
• When and Where To File: Mail within 3 days of your return being accepted by the IRS
• Mail Form 8453 to: Internal Revenue Service Attn: Shipping and Receiving, 0254 Receipt and Control Branch Austin, TX 73344-0254

You will check the Box on Form 8453 for Form 8283:

Click here for Publication 526 (2024), Charitable Contributions

Click here for Instructions for Form 8283 (12/2024)