There are two ways to add the Polygon main network to MetaMask.
1. Through Polygonscan
2. From within Metamask (manually)
In both cases, you will need to access the official Polygon sites.
Adding a network through Polygonscan
-Go to the website polygonscan.com
-Scroll down to the bottom of the website and click on the "Add Polygon Network" button.
After clicking on the "Add Polygon Network" section, the following Metamask notification will appear:
Click on "Approve". The addition of the network will then be finalized and you will be able to access the main Polygon network on your MetaMask.
If this method does not work, you can add the network manually from within MetaMask itself.
Adding the MetaMask network (manually)
In MetaMask, click on the button that reads "Ethereum core network". This will display the networks you have previously installed in your MetaMask. To add a new network, you must click on the "Add network" button.
You will then be presented with the following window, in which you will have to fill in the data manually:
The details for adding the network manually can be found at the following official link from the Polygon developers: https://wiki.polygon.technology/docs/develop/metamask/config-polygon-on-metamask/.
In the following example, the fields were completed as follows (it is always advisable to access the official sites of the respective networks to check the information you see on their websites):
Network name: Polygon Mainnet
New RPC URL: https://polygon-rpc.com/
String identifier: 137
Currency symbol: MATIC
Block explorer URL: https://polygonscan.com/
Once you have filled everything out, click on “Save”. MetaMask will then give a notice that the network was successfully been added and, from that moment on, Polygon's main network will appear in the drop-down list of networks in your MetaMask.