When you click on the Interseller button on a contact's LinkedIn profile, you can select the email address you want to save. If they're a contact that's been previously added or messaged on Interseller before, you'll see that in the highlighted information. This is especially helpful for checking to see if a contact exists as a duplicate in other sequences.
If you have an additional custom fields for the sequence that you'd like to fill in for the contact, you can scroll down to add it in.
Once you've save the contact's email to a sequence, you can customize your initial message for the contact within their LinkedIn profile.
You can always edit your personalized messages for contacts in the preview tab of the sequence.