This can be done without using an iframe or a plugin. Simply use the code block.
Please access this trial site before this expires:
Link: https://circle-bell-h2l2.squarespace.com/
password: demo
If the trial website from the link above expires, you can see this Wordpress demo link and it works the same way on how I setup Bullhorn Squarespace integration :https://cjcortez.com/bullhorn-wordpress-integration-demo/
Contact me on Fiver for better understanding on this topic: https://www.fiverr.com/s/zw102E