Entity Framework - get related items without explicit reference using Fluent API configurations

Let's say I have database table A with optional property X. I have table B with a composite key (Y, X). These two tables are not explicitly related in any way.

e.g.

create table A
(
    Id uniqueidentifier not null,
    X uniqueidentifier null
)

create table B
(
    Y uniqueidentifier not null,
    X uniqueidentifier not null,
    Constraint PK_B Primary Key (Y, X)
)

A and B are implicitly related via property X, in the sense that X is an orphan id (its base table has been moved out)

For my model A in C# code, how can I populate IList<B> using property X to get all items in B containing X? Ideally, I'd like to use Fluent API configurations.

e.g

public class A {
    public Guid Id {get; set;}
    public Guid? X {get; set;}
    public IList<B> RelatedItems {get; set;}
}

public class B {
    public Guid X {get; set;}
    public Guid Y {get; set;}
}

How do I configure my entities using Fluent API so that I can use something like

var list = await context.AQuery()
.Include(a => a.RelatedItems)
.ToListAsync().ConfigureAwait(false);

or any other way to populate IList in A

Thanks in advance!