Digital certificate

I need to get a digital certificate for issuing electronic bills with a
signature.

According to the instructions in the field "serial number" must go the
initials RUC followed by the tax id number, but in the DCM there is no
field for the "serial number".  When I requested a digital certificate
the first fiels is "Common Name",  how can I give it the "Serial Number"?

From the manual (original in Spanish): En el caso de optar por el

certificado de persona jurídica, el RUC del contribuyente estará
contenido en el campo SerialNumber. 

Here is a sample from Documenta:

Subject Name
serialNumber (Serial Number):    RUC 80050172-1
CN (Common Name):    CA-DOCUMENTA S.A.
O (Organization):    DOCUMENTA S.A.
C (Country):    PY



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