1) create 2 bson values:
BsonValue convertedValue1 = BsonString.Create("str1");
BsonValue convertedValue2 = BsonString.Create("str2");

2) Create BsonArray: var array = new BsonArray(new[]

{convertedValue1, convertedValue2}

);
3) Create query: Query.ElemMatch("Properties", Query.And(Query.EQ(propertyIdFieldName,"some Id"), Query.In("someProperty", array )));

Generated result:
{ "Properties" : { "$elemMatch" : { "PropertyDefinitionId" : "some Id", "someProperty" :

{ "$in" : [["str1", "str1"]] }

} } }
Expected result:
{ "Properties" : { "$elemMatch" : { "PropertyDefinitionId" : "some Id", "someProperty" :

{ "$in" : ["str1", "str1"] }

} } }