Issues with Modal Box

Hi there - I am trying to include a popup window that contains a third party application (a decision tree designed on the zingtree site). I have copied the html code from their website to set up the button / popup window, and pasted this into an HTML object placed on the page.
The issues that I am having are that the pop up window will only size to whatever size I drag it to in the Editor (it sizes automatically when I trial it on the zingtree website), it’s position is not centred on the page, it just opens in the location I place the original HTML object, and the click button just is positioned top left of the popup window (related to the pop up window no centering on the page). Can somebody please help with this? I am not a web designer so please be gentle!