Log in to your Google Analytics Account and perform the following tasks:

1. In Google Analytics, click  Admin (lower left).
2. In the Account column, make sure that your desired account is selected. (If you only have one Google Analytics account, it's already selected.)
3. In the Property column, select the GA4 property that currently collects data for your website.
4. In the Property column, click Data Streams. It's the first option in the Property column.
5. Select your domain from the list of data streams.
6. You will find your MEASUREMENT ID next to STREAM ID.