Combine multiple base table attributes to form a DynamoDB GSI PK/SK

I am trying to create a GSI on a DynamoDB Table using AWS console by choosing PK or SK as combination of multiple base table attributes. However, none of the below formats are working for either PK or SK. Please guide me where I am doing wrong ?

1. <custid>#<userid>
2. [custid]#[userid]
3. CUST#<custid>#USER#<userid>
4. CUST#[custid]#USER#[userid]

Base Table has attributes: id (PK) (string), custid (string), userid (string)

#2 - Reference link(s): https://repost.aws/knowledge-center/dynamodb-create-composite-key https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/bp-sort-keys.html

#1,#3 - Reference link(s): https://www.alexdebrie.com/posts/dynamodb-one-to-many/#denormalization-by-using-a-complex-attribute:~:text=ORG%23%3COrgName%3E%23USER%23%3CUserName%3E