Hi Jill T!
Here is a link to the NUCC's CMS1500 claim form instructions:
https://nucc.org/images/stories/PDF/1500_claim_form_instruction_manual_2021_07-v9.pdfPage 23 supplies the details for field 14, and the details for field 15 are found on page 24.
Based on these instructions, it looks like it may be:
Field 14:
qualifier 431 with date of accident/injury (8/11/2021)
Field 15:
qualifier 439 with date of accident (8/11/2021)
Also be sure to complete field 10 for the accident as well!
Hope this helps!