What have you got from them, is it a chunk of html code like here?
https://help.klaviyo.com/hc/en-us/articles/115005085167-Export-the-HTML-for-an-Email-Template
https://help.klaviyo.com/hc/en-us/articles/360006897412-Where-to-Paste-a-Form-s-Embed-Code
https://help.klaviyo.com/hc/en-us/articles/360002035871-Install-Klaviyo-Signup-Forms
If so, then you need to be adding it to an iFrame on your page.
https://support.wix.com/en/article/using-iframes-to-display-visible-content-on-your-site
Or as a new custom tool in Tracking and Analytics.
https://support.wix.com/en/article/embedding-custom-code-to-your-site
Please note that as stated on the linked page above that Wix does not provide support for code from a third party provider as like in this case.
***Wix cannot provide support for external codes that were not created or tested with Wix. If you are experiencing an issue with your code snippet, please contact the provider directly. ***
For more help with this you should be going back to the third part providers own support, which in your case is here.
https://help.klaviyo.com/hc/en-us