If you're using a screening question based on which you'll decide the treatment, then you're best option would be to use branch logic.
If you're just looking for proportional assignment, then a randomizer should do the job.

Thanks Ahmed for your suggestion, the screening question is only to identify the sexes of the participants so that they can be split into groups i.e. male or females, which will then be randomly assigned into the (6) different treatments (via randomiser) i.e. to achieve a close to equal split in sexes across the conditions.
Would it be appropriate to create three branch logic (i.e. if male/female/others), then randomly present 1 of the 6 treatments (via the randomiser)?