What is the code to add a text box and a button to my website

When you hit the button I want it to open what's in the text box as a url using the website's url, for example codingforums.com/TEXTINBOX

at the end of the TEXTINBOX I need to know how to add .htm after the string in the box.

I know this code is probably pretty simple but I can't seem to find it online.

I want it to open the link in the same/self window.