If you want an API 610 check, fine, you can do manually or you can apply a force in opposition with your thrust in order to emulate the effective load on the nozzle. Alternatively you can do as follows. Do not fill in the “Effective ID” field then calculate the real force on the nozzle and insert it in the “Force/moments” field.
However you will be responsible for the fact that you apply a thrust on the pump case. If the pump vendor is informed and accept this, fine. If not, do not use untied bellows.
Note the force in Anchor A from your example.

Hope I've made myself clear.

Regards,