<script src="..."> that pulls in the library, and a short block of inline JavaScript that configures it. Sleekplan keeps those two parts in separate fields, so you paste each one where it belongs.
Custom code is available on the Pro plan and above. The inline Custom JavaScript field also requires a custom domain, so your board runs on an address like
feedback.yourcompany.com. On a free .sleekplan.app address the loader in the head still works, but the inline configuration is saved and ignored.Add Google Analytics
Google Analytics 4 gives you a two-line snippet. The first line is the loader, the second is the configuration. Split them across the two fields as shown below.Open the Portal settings
Go to Settings → Public Portal & Widget → Portal and scroll to the Custom code section at the bottom of the page. Click it to expand.

Paste the loader into the head markup
In the Replace 
<head> markup field, add the Google tag loader. Keep the full <script> tag here, because this field accepts external script references.G-XXXXXXXXXX with your own Measurement ID.
Paste the configuration into Custom JavaScript
In the Custom JavaScript field, add the inline setup. Do not wrap it in a
<script> tag: Sleekplan adds the wrapper for you, so pasting your own would break the code.Adding other trackers
The same split works for any third-party snippet that arrives as a loader plus an inline block, such as a product-analytics or chat tool. Put anything that looks like<script src="..."> in the <head> markup field with its tag intact, and put the inline configuration (the code that would normally sit between <script> and </script>) in the Custom JavaScript field without the tags.
For a full reference on every field, what each one accepts, and the custom-domain requirement, see Add custom code to your portal.
Related
Add custom code to your portal
The complete reference for the head markup, CSS, and JavaScript fields.
Set up a custom domain
Required before inline Custom JavaScript runs on your board.
