Anyone know if it is possible to add a Workflow button to a specific Sales Order Form? i.e. not all Sales Order forms

Add a condition on the workflow action for Custom Form = your custom form

@michoel fantastic, thank you. is there also a condition that can be added to disable or hide the button if the workflow has already run?

Probably the easiest way would be to add a custom checkbox field to the record and have the workflow update that when it runs