Tag Manager is used when the advertiser has integrated their website with Google Tag Manager and uses that to add any pixel directly on their website.
When You are adding pixel on advertiser website via Tag Manger, make sure you have white labeled the Tracking Domain, click here to learn how?
You need to be ready with Pixel for default and Goal, you can check here to learn how you can generate pixel for Default and Goal.
Once you have generated pixel share with the advertiser and they can follow steps below to add pixel.
Open Tag Manager with which you have integrated your website and there you will find an option for tag
Once you click on Tags, a side drawer will open Up there click on edit Icon on "Tag Configuration".
Once you click on Edit, a side drawer will open Up there scroll down and select "Custom HTML"
In the HTML box place the Trackier iframe/image Pixel, Now an important part is to define where this code should trigger so that needs to be defined in the "Triggering" section.
Same way Add multiple pixels for different Goals you want to track and select the Trigger as per need.
Pixel need to be placed on Thank you page inside <body> tag. Thank you page will be the page which pop up when user fill up lead form, or made the purchase.
The final step is to make the changes live, click on the "Submit" button and after some time pixel is added then you can do the Pixel test