@kiruthika_s, the partitionName field isn't considered a co-lookup key in the POST request with the Email Address field. AFAICT, the Marketo REST API sees the entire lead database as one unit while making updates, not the virtual partitions created by creating the person partitions via Marketo > Admin. Also, with the default settings, duplicates aren't allowed in the person partitions. However, if you have intentional duplicates with partitions and workspaces, it is required to have the secondary dedupe field as leadPartitionID. Once you have that setup, you can use the leadPartitionID as the lookupField similar to email and Person ID (see the JSON payload below).
{
"action": "updateOnly",
"lookupField": "leadPartitionID",
"input": [
{
"email": "a@example.com",
"leadPartitionID": "1",
"firstName": "Luke"
}
]
}
Check out this article on Marketing Nation: https://nation.marketo.com/t5/employee-blogs/duplicates-with-partitions-and-workspaces-via-rest-api/ba-p/325790