The URL/Audience tab is the second one you are presented when accessing the Experience Manager.
If you are on the default Experience for a page, you will see the URL tab. From this tab, you can choose the publishing method of your choice.
If you select the Custom Domain option, you will see the following screen:
If you want to learn more about adding a custom domain to your Instapage account, you can find a step-by-step article here: Adding and Managing Custom Domains
If you choose the WordPress option, you will have to install the plugin as described here: Publishing your page to WordPress
If you choose Drupal, you will have to install the plugin as explained here: Publishing Your Page to Drupal
If you're still in the testing phase, you can choose pagedemo in order to have a live page without adding the permanent URL yet.
Clicking Save will only save the configuration. The actual act of publishing happens when you click Publish. If you want to change the publishing method you can click on Switch Method and you will be brought back to the Publishing Options screen.
If you are on a non-default Experience, you will see the Audience tab.
If you want to learn more about UTM parameters and how to use them, as well as how they interact with our experiences, check out this blog article.
Within the URL field, you will see the URL that you have published the default experience to. If you have not set it up yet, you will see a message and the option to go and set it up.
If you want to add custom parameters to your URL you can do so by clicking the button on the lower left side.
You will then be presented with two fields that you need to fill out, "Name" and "Value". To delete a custom parameter you can click the "X" at the end of the row.
The URL displayed at the bottom of the Audience tab will dynamically readjust itself as to reflect the changes made in the fields above.
After the Audience URL is set you can Save it. After saving it, to push it live you need to click on the Publish button in the upper right corner.
Please see this article for the next tab of the Experience Manager: Page Experience: Integrations