access sequence sap crm

Hi Gurus,

I need to create an Access sequence for a specific TAX requirement. But I need to set the sorce field with a Constant and Initial Value into the first source field, but when I try to do that. The system send me the next messsage:

"You cannot maintain a source field name for an initial constant"

Anybody knows if I have the possibility to set an initial value or a constant into an Access sequence in this way? or do you have suggestions to set constants?

Thanks in advance.

Maria.