Being able to use multiple recruitment services for the same project is really vital for us.

E.g. we often need to recruit students through SONA and non-students through other means (e.g. direct links on Facebook). Currently it is not possible to do both for a single project, as enabling the SONA recruitment service means that direct links no longer work (as no ID's are generated).

There are many ways to solve this, but one would be to designate a reserved query parameter ID value to let Gorilla know that an ID needs to be generated. For instance:

?external_id=none

Secondly to allow for proper redirects, this (or another query parameter) should be accessible as a branch property to select the relevant finish node.