what was unsuccessful about your workflow? it seems like that should work. you might also be able to do a non-stored field with a default formula value of internal ID, bypassing a workflow entirely. haven't tried it though so it might not work depending on weirdness w/ internal ID specifically or something

I like the suggestion of a field formula to set the value to the internal id. However, when I tried {internalid}, the project task form shows an error. Field not found.

try {id}

@KevinJ of Kansas thanks! That worked. And thanks @amy for the suggestion. I appreciate the help.