I found the problem, the header of the SDP frame should start with 0x83 which is "type" 3 not just 3
So writing to the packet-memory (starting from 0x00)
0x83 1 25 64 65 66 67 68 69 0 0 0x30 0x31 0x32 0x33 0x34
(never mind the rest of the registers as long as they are zero)
And then enable it in the main map by setting 0x40 to 0x40
will send an SDP (to adv7842)
SDP infoframe type 3, version 1, crc 0x1a, len 25
Vendor Name: ABCDE
Product Description: 1234
Source Device Information: unknown (0)