How to add custom claims to access token in IdentityServer4? [closed]

You should implement your own ProfileService.
Have a look in this post which I followed when I implemented the same:

Extending Identity in IdentityServer4 to manage users in ASP.NET Core

Here is an example of my own implementation:

public class ProfileService : IProfileService
{
    protected UserManager<ApplicationUser> _userManager;

    public ProfileService(UserManager<ApplicationUser> userManager)
    {
        _userManager = userManager;
    }

    public async Task GetProfileDataAsync(ProfileDataRequestContext context)
    {
        //>Processing
        var user = await _userManager.GetUserAsync(context.Subject);

        var claims = new List<Claim>
        {
            new Claim("FullName", user.FullName),
        };

        context.IssuedClaims.AddRange(claims);
    }

    public async Task IsActiveAsync(IsActiveContext context)
    {
        //>Processing
        var user = await _userManager.GetUserAsync(context.Subject);
        
        context.IsActive = (user != null) && user.IsActive;
    }
}

Don’t forget to configure the service in your Startup.cs (via this answer)

services.AddIdentityServer()
    .AddProfileService<ProfileService>();