• ds-lite https://tools.ietf.org/html/rfc6333
• map-t https://tools.ietf.org/html/rfc7599

VAT commands that I’ve tried
vat# map_add_domain ip4-pfx 20.169.0.0/16 ip6-pfx 2001::/24 ip6-src 2001:db8:ffff::1 ea-bits-len 40 psid-offset 6 psid-len 8 map-t
map_add_domain error: Unspecified Error
vat#

VPPCLI command
root@DUT-1:/home/lab# vppctl map add domain ip4-pfx 20.169.0.0/16 ip6-pfx 2001::/24 ip6-src 2001:db8:ffff::1 ea-bits-len 40 psid-offset 6 psid-len 8 map-t
root@DUT-1:/home/lab# vppctl show map domain
No MAP domains are configured...

DBGvpp# map add domain ip4-pfx 20.169.0.0/16 ip6-pfx 2001::/24 ip6-src 2001:db8:ffff::1 ea-bits-len 40 psid-offset 6 psid-len 8 map-t
map_create_domain:164: MAP-T only supports ip6_src_len = 96 for now.

The 'ip6-src' parameter has to be a prefix (and it has to be a prefix of /96) the remaining 32 bits are used for the outside IPv4 source address.