Just build the string manually, and then execute it:
DECLARE @SQLString nvarchar(1000)
DECLARE @ParmDefinition nvarchar(1000)
DECLARE @keyIndex nvarchar(100);
set @keyIndex = 'Id'
SET @SQLString =
N'CREATE FULLTEXT INDEX ON dbo.objects
(
[Name] Language 1033,
[Description] Language 1033
)
KEY INDEX ' + @keyIndex +N'
WITH STOPLIST = SYSTEM, CHANGE_TRACKING AUTO;';
EXEC(@SQLString)