Set Variable in Jinja

I would like to use a dummy variable in a pipeline to store the parent Purchase Order ID. If existing, use the existing ID, if not create a new PO and use the new ID.  The pipeline would then continue, adding child records to the "poholder" ID.

I haven't used dummy variables before and wanted to check if the formatting is correct?  And if I can just nest this statement within another text field.  

If not existing:

 {% set poholder = c.id %}

If existing:

{% set poholder = b.id %}

Thank you!