To connect Gmail mailbox to our warmup tool, you can follow these steps:
Enable 2-step verification for your Gmail account.
Go to My Account > Security > 2-step verification:

After enabling 2FA, you will now be able to create an App password for your Gmail.
Go to My Account > Security > 2-step verification > App password > Create
Or go to this link: https://myaccount.google.com/apppasswords

Now, use the generated app password to connect your mailbox to E-Warmup:

Tips: You can also connect your Google Workspace (G Suite) using these 3 steps.