This article explains how to create your Splash Page from default one, assuming that you've already an SSID with Splash Page Editor (as here described How to create an SSID )
How to create a new Splash Page from default one
In order to edit the default splash page with the Splash Page Editor, follow these steps.
STEP 1
In your Classic Hotspot Cloud Management, click on " WiFi SSID " then click on "Splash Page" tab

STEP 2
You will see that the SSID refers to this page: "Default splash page", or to another page that you have selected and applied.
Click on "Edit Splash Page".

STEP 3
In the Editor, work on the Default Splash Page. Any change to this page will be automatically applied an saved.
Note that you are not creating a new splash page, but you are modifying the default splash page.
Related Article
How to create a new splash page