This was resolved by contacting technical support.
My support person said that I can put an expression a[b][c, d] on a link by
WSPutType(link, WSTKFUNC);
WSPutArgCount(link, 2);
WSPutFunction(link, "a", 1);
WSPutSymbol(link, "b");
WSPutSymbol(link, "c");
WSPutSymbol(link, "d");
The function WSPutType() shows the type of the object to be put on the link. The type can be
WSTKERR,
WSTKINT,
WSTKFUNC,
WSTKREAL,
WSTKSTR,
WSTKSYM
And so on.
More complex cases require further study, but I got a clue.
Thank you, technical support.