Issue #2302 was marked as a duplicate of this issue.
@yeshasvitvs thanks for your comment in #2302. The Graphical ApplyForce tool uses the Link::AddLinkForce function that was added to ODE in pull request #1456, but doesn't have an implementation for DART. DART does have AddForce, which you used in your plugin, but it needs this additional API to be implemented.
@mxgrey is taking a look at this (I couldn't get the Assign field to work for him)