External frontpage / then WordPress

Hi folks!
I would like to have a frontpage uploaded from an external HTML.

I edit the .htaccess file to give priority to index.htm and then index.php (the home of WP).
It works, but I get this message:

{\rtf1\ansi\ansicpg1252\cocoartf1038\cocoasubrtf360 {\fonttbl\f0\fswiss\fcharset0 Helvetica;} {\colortbl;\red255\green255\blue255;} \paperw11900\paperh16840\margl1440\margr1440\vieww9000\viewh8400\viewkind0 \pard\tx566\tx1133\tx1700\tx2267\tx2834\tx3401\tx3968\tx4535\tx5102\tx5669\tx6236\tx6803\ql\qnatural\pardirnatural \f0\fs24 \cf0 \ \ \ \ \ \ \ }

Am I doing it in the right way?
And if yes, how can I hide/delete this error?

PS: and if I would like to have the first page with a button that slides up all the content and reveal the index.php (frontpage of WP) like this?
Should I edit the page of WP or can I upload it dynamically after pressing the button?

Thank you!