How to query(Where) on generic type with just a string name

I want to create a generic methods to handle the unique fields in table.
for example I have 2 unique field, and I want to when I call _entities.Add() automatically checks the 2 unique fields that there is no records in the database with that record. I have this entity

[Index(nameof(Product.ProductNumber), IsUnique = true)]
[Index(nameof(Product.Url), IsUnique = true)]
public class Product
{
    public int Id { get; set; }

    [Required]
    [MaxLength(50)]
    public string Title { get; set; }

    public int ProductNumber { get; set; }

    [Required]
    [MaxLength(50)]
    public string Url { get; set; }
}

Now this is my generic service

public class GenericService<TEntity> : IGenericService<TEntity> where TEntity : class
{
    private readonly DbSet<TEntity> _entities;

    public GenericService(ApplicationDbContext dbContext)
    {
        _entities = dbContext.Set<TEntity>();
    }

    public bool Add(TEntity entity)
    {
        List<string> uniqueFieldsOfEntity = entity.GetType()
            .GetCustomAttributes<IndexAttribute>()
            .Where(x => x.IsUnique)
            .Select(x => x.PropertyNames.FirstOrDefault())
            .Where(x => x != null)
            .ToList();
        foreach (string uniqueField in uniqueFieldsOfEntity)
        {
            // HERE how can I work with Any with just a string
            //var hasAlready = _entities.Any(x => x.uniqueFiled == entity.uniqueFiled);
            if (hasAlready)
                return false;
        }
        _entities.Add(entity);
    }
}