Open Setup → Email Template and click on the mail where you want to add the Auto Login Link.
At the WHMCS Mail Editor you can add &{$modautologin} to existing links or create a new link with &{$modautologin} at the end.

URL Examples:

If you use a mail variable, which is already sent as a hyperlink by default, please add "|strip_tags:false" to the WHMCS mail variable. This will remove the hyperlink from the inbuilt WHMCS variable.

Please take a look at the last example above.

Note: Replace & with ? if there are no parameters before the Auto Login Link (like demonstrated in the examples above).