Add property-ref and not-found attributes in mapping by code

I cannot find the way to indicate what was done with the "property-
ref" and "not-found" attributes in a ManyToOne() relationship.

Eg:
<many-to-one
name="Client"
column="cCode"
property-ref="ClientCode" // the non-id column to use in the join
not-found="ignore"
/>

ManyToOne(x => x.CardHolder, map => {
map.Column("sClient");
map.PropertyRef(x => x.ClientCode); // No
PropertyRef() method available?
map.NotFound(NotFoundOptions.Ignore); // No NotFound()
method available?
});

Unfortunately I'm dealing with legacy databases and this issues come up to often.