Hello Khalid,
Firstly you will need to create a login page using the login widget of the ServiceNow.
Once you are done with creating the page then you can folow below steps to set it as your login window:
1. Go to Service Portal->Portals
2. Go to the portal you have created.(in the below example i have gone to the default SP)
3. In the screenshot, you can see there is a field 'login page'.
4. You change the page to your custom built login page.
Kindly use the below links for referrence:
Create a Page - ServiceNow Developers
Login widget
Kindly mark as correct if it helps.
BR,
Nayan
