.net and sqlserver - Why do I get a login fail error

I have a web app built in VS2019 using asp.net framework 4.8 that deploys to Windows Server 2008 2R

On my machine, my con string is like so:

data source=MySqlServer8;initial catalog=DbTest;integrated security=True;MultipleActiveResultSets=True;App=EntityFramework

and I have a DbContext with the right initialisers etc.

This all works fine in development using windows auth because I have an admin account on the server, but when I deploy this app to the server it doesn't connect; obviously my account isn't being used because it's running on an AppPool in IIS, so I originally tried creating the specific app pool as a user IIS AppPool\MyAppName - that didn't work because I think the server and IIS are running separately.

After digging into exception > inner exception > inner exception I spotted this:

Login failed for user 'POPDom\MySqlServer8$'.

So I created POPDom\MySqlServer8$ as a windows user with all roles selected, on the database DbTest ... but I'm still getting the error.

I also tried creating the user MySqlServer8$ without the POPDom domain but it said it wasn't a valid name or I didn't have permission. Is that the account I should be targetting, without the domain name?

I can probably create a new account with a password to connect but why does my new 'POPDom\MySqlServer8$' account not work? I'd love to use just windows authentication and not have passwords in my web.configs