When you set up a new Property in Google Analytics 4 to measure your website’s performance, GA4 creates the Measurement ID which identifies your property.
This is unique and it’s different for every property. All measurement IDs begin with the “G” letter, following by ten randomly numbers and letters.
Furthermore, remember that every Data Stream has its own Measurement ID.
Where can I find my measurement ID?
You can simply go to the Admin section and follow this path: Admin > Data Streams > select your data stream (if you had more than one).
Once you followed the path you’ll find it, as you can see in the image below and you have the possibility to copy it, by clicking the image close to the number: