You'll have to turn on your Hosting mode first, JoPri.

Then, you can start connecting your computer to the Host server.

Just a heads up, you should turn this on only for the Host computer. If you're using a server to host your file, one computer still has to act as a Host. You must not turn on hosting for workstations.

Here's how to turn it on:

1. Go to the File menu, and then select Utilities.
2. Select Multi-User Access.
3. Click Yes to confirm.

Once turned on, please follow these steps:

1. Go to your server computer (a computer where you store your company file).
2. Click the Start menu, and then type Database.
3. Select QuickBooks Database Server Manager.
4. If you can't see your folder location, click Browse, and then choose your company file.
5. Click OK.
6. Select Start Scan. This will repair your firewall permission to allow QuickBooks to communicate your network.
7. Click Close, and then try opening QuickBooks in Multi-user mode on your workstations.

I've added an article here to know more about Hosting your computer: Host your company data file in multi-user mode.

These details will help you connect your other application to the hosting computer. I'm always around if you need more help.