var bannerContainer = document.getElementById('fwrBannerContainer');

var htmlStr = '<iframe style="margin: 0; padding: 0;" width="300" height="250" src="https://www.forces-war-records.co.uk/adscript/namesearchform300x250/?adType=namesearchform_300_x_250&av=bigen" scrolling="no" frameborder="0" />';

bannerContainer.innerHTML = htmlStr;