Have a look at the following URL. https://documentation.meraki.com/zGeneral_Administration/Cross-Platform_Content/Configuring_a_Custom...
They explain in detail how to use your own splash page. By the looks of it, the textboxes is still a username and password that gets submitted to the meraki portal.
I use purple wifi togeth with Meraki as my guest portal, very easy to use and very customizable.
And you get a free license if you have less than 5 AP's. or the 7 day trial for more. try it out you wont be disapointed https://purple.ai/
@Wickus is talkign about a fully custom splash page that you host yourself.
I think you may be talking about the customizing of the Meraki hosted splash page? For those you get access to the source code (html) and the style sheets (css). You can also upload your own images to use in the code. What you don't get access to is the php code in the backend. Here's an example: