In some cases you may have created your own custom return to work workflow, in this process you'll want to send out your return to work form however when creating the workflow step to do so you may notice that your Return to Work form is not available in the dropdown:

This is likely because the Return to Work form was made using a 'Return to Work' type of form, these are not designed to be used in a workflow and therefore are not selectable at this stage. If you instead create an Other type form using the same fields as your return to work form you'll be able to choose the form in this workflow step.