Entity Framework join with a partially hardcoded "on" due to key value pair?

A Where clause should suffice:

db.Table1
    .Join(db.Table2, t1 => t1.id, t2 => t2.id, (t1, t2) => new { t1, t2 })
    .Where(x => x.t2.attributeId == 123);

Ideally Table1 should have a navigation property to Table2: (either HasOne or HasMany)

Singular Table2:

var result = db.Table1
    .Include(x => x.Table2)
    .Where(x => x.Table2.AttributeId == 123);

or for a collection of Table2s...

var result = db.Table1
    .Include(x => x.Table2s)
    .Where(x => x.Table2s.Any(t2 => t2.AttributeId == 123);

which would return any Table1 containing a Table2 with that attribute...

or with a collection that you want to filter the Table2s:

var result = db.Table1
    .Where(x => x.Table2s.Any(t2 => t2.AttributeId == 123)
    .Select(x => new 
    {
        Table1 = x,
        FilteredTable2s = x.Table2s.Where(t2 => t2.AttributeId == 123).ToList()
    });