1. Add the [DbFunction] attribute to your function signature, specifying the function name and the schema name:
[DbFunction("newid", "dbo")]
public static Guid NewId()
{
return Guid.NewGuid();
}

2. In your DbContext, register the function using the modelBuilder:
modelBuilder.HasDbFunction(typeof(MyFunctions).GetMethod(nameof(MyFunctions.NewId)));

3. Use the function in your LINQ query, just like any other SQL function:
var data = dbContext.MyEntities
.Select(e => new {
Id = e.Id,
Guid = MyFunctions.NewId()
});

4. If you want the function to be called automatically by EF-Core when you call SaveChanges(), you can create a trigger in your SQLite database to call the function:
CREATE TRIGGER newid_trigger
AFTER INSERT ON MyEntities
BEGIN
UPDATE MyEntities
SET Guid = newid()
WHERE rowid = new.rowid;
END;

This approach allows you to use existing SQLServer code and consume SQLite functions in your DbContext, without having to create a new SQLiteConnection.