You can use the following variable substitution pattern (please use curly braces, not parentheses):
{FIRST_NAME} - First Name field
{LAST_NAME} - Last Name field
${custom1} - Custom Variable 1
${custom2} - Custom Variable 2
${custom3} - Custom Variable 3
${custom4} - Custom Variable 4
${custom5} - Custom Variable 5
${customN} - Custom Variable N
Custom Variables can only be accessed with the Corporate and higher licenses.