Hard one | extracting dynamic field-name

How To
1

My scenario first block is custom webhook, 12 different possibilies can pass data to it
DOUCMENTS_P, DOCUMENTS_K, etc…

for example, sometimes the webhook can receive this payload:
{
“DOCUMENTS_P”: {
“DOCNO”: “GR24000199”
}
}

and sometimes it can receive this payload:
{
“DOCUMENTS_K”: {
“DOCNO”: “GR24000199”
}
}

please pay attention to the difference between JSON’s
one has field-name DOUCMENTS_P and the other one has DOCUMENTS_K field-name

how can I extract the field-name instead of re-mapping 12 different cases?

thanks!