How to pass values of Multiselect as arrays to controller.

My entity class
-----------------------------------

public class AssignCategories
{
[Key]
public int AssignId { get; set; }
public string CategoryId { get; set; }
}

Create controller
-----------------------------------

[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Create([Bind("AssignId,CategoryId")] AssignCategories assignCategories, string[] selectedValues)
{
ViewBag.SubCategories = new MultiSelectList(_context.ParentCategories, "ParentCategoryId", "Name", selectedValues);
if (ModelState.IsValid)
{
_context.Add(assignCategories);
await _context.SaveChangesAsync();
return RedirectToAction(nameof(Index));
}
return View(assignCategories);
} 

Html Tag Helpers
-----------------------------------
<form asp-action="Create">
<div asp-validation-summary="ModelOnly" class="text-danger">div>
<div class="form-group">
<select asp-for="CategoryId" class="form-control" asp-items="ViewBag.SubCategories" multiple id="MultipleSelection" >select>
<input type="submit" value="Create" class="btn btn-default" />
div>
div>
form>
I need to pass multiple values to database
-------------------------------------------



When I click on create it only add only one record
---------------------------------------------------