Add Roles to User using checkboxes

I am developing a Razor application and I need the Admin to assign or update roles of the Users. I am using a checkbox to carry out this action. So far, I have been able to populate the view to show the users and their respective roles ticked in the checkbox, but I haven’t been able to update their roles, as anytime a checkbox is ticked to add a role, it doesn’t add the role to the user. I think this line is the culprit: var selectedRoles = model.Where(x => x.Selected).Select(y => y.RoleName);.

How to make bootstrap alert popup in another page when clicking on a button

Im trying to make an admin page where if i click on the button ‘send’, the message will be sent and there will popup a bootstrap alert on the employee page.
I have already the bootstrap alert setup, the message for the first time is shown. After I close and i send a message again i dont see any alert. I think it needs to be triggered when i click on the send button but im struggling about how to do that. Hope anyone can show me a simple way.
Here is the code for the admin page to send: