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Your Salesforce security token should have been emailed to you when you set up your Salesforce account or the last time you reset your password. If you are unable to find this email, you will need to reset the token.

Click on your profile picture > My Settings > Personal > Reset My Security Token.
An email will be sent to you from support@salesforce.com with the subject of "salesforce.com security token confirmation.
If you do not see the option to reset your token, you may be using IP Login Restrictions. Because this restriction disables the use of security tokens, organizations with this in place will not be able to sync with Lead Liaison. You may, however, be able to place an exemption for the Lead Liaison profile, allowing for the generation of the token. To check this setting in Salesforce, navigate to Setup > Security > Network Access.

Resetting your security token will break any existing integrations with your salesforce.com account. You will need to update your security token in any other integrations that requires it.