I am receiving a string from a C# program that values are seperated by a + sign.  I have a formula that I need to use, but I'm unsure of how to make it recieve multiple values.  The formula is:

If Count ({Command.COMMERCIAL_COUNTER}, {Command.COMMERCIAL_COUNTER}) >1
and {Command.CS_GROUP_EXTERNAL_CODE}= {?ReportCopySplit}
then
{Command.CS_GROUP_EXTERNAL_CODE}={?ReportCopySplit}
Else If
Count ({Command.COMMERCIAL_COUNTER}, {Command.COMMERCIAL_COUNTER}) >1
and {Command.CS_GROUP_EXTERNAL_CODE}<>{?ReportCopySplit} 
then
{Command.CS_GROUP_EXTERNAL_CODE}= "NAT_000" 

How do I make my parameter accept the string?  I have the paramater set to accept multiple values and it is set to a string.