Replace text in body with text from an input textbox

I have an html page where it contains a serious of codes, some of those words are variables and i have a series of fields that an user will enter, for example, a field for a switch hostname, in the body it says HOSTNAME, the purpose is for a user to enter the value in one textbox and I want all occurrences in the body where it says HOSTNAME to be replaced by the word entered in that textbox. There are more textbox input fields for other words.

I would greatly appreciate it if you can guide me in the right direction.

The challenge: i can't use PHP or other files because where this will be allocated only supports HTML