Adding iframes

Hi there - I'm going round in circles and hope the circle forum will be able to help!!!

I'm trying to connect a site to a 3rd party using some custom code (its for a booking availability for bnbs).

I tried just putting the html just in a code block or to embed it - but it doesn't render well.  So I added some custom css but this is conflicting with the code so although it looks better it doesn't work properly.

I've been advised by the 3rd party (who don't support) that I should use an iframe by creating a blank html page, inserting the code, and then in an embedded widget link to that html page.  However, I can't create a blank HTML page on a squarespace site?  Is this correct.

I am therefor really struggling with what to do.  Can anyone help:

Custom CSS that is conflicting:

.guesty-root-element {
    justify-content: center;
}
section#guesty-search-widget__datepicker input {
    max-height: 18px;
}

website is apom.squarespace.com and the password is test

@brandon  I wonder if you might be able to help as you have answered some similar questions?

Are you trying to center the search interface within the page and then tweak the date picker height?

If so, for centering the search interface you should wrap it in another div and then center the search within that. 

<div id="search-widget-wrapper"><div id="search-widget">..the script..</div></div>

Then center it using the following inserted into the custom css area

#search-widget-wrapper {
	width: 100%;
	#search-widget {
		margin: 0 auto;
	}
}