question

User commands are stored in the tree under MODEL:/Tools/UserCommands/rotate. Their code is stored in the first subnode which is usually called "code".

You can use the nodefunction command like you would normally to execute the user command, from the DLL:

treeonde rotateCommand = first(node("MODEL:/Tools/UserCommands/rotate"));
double result = nodefunction(rotateCommand, robot, Rot, order_number, table_name1);

Note, however, that this makes your DLL non-reusable by another model, unless that model also happens to have the same user command.

You may want to consider implementing the rotate functionality in the DLL instead:

visible double rotate(FLEXSIMINTERFACE) {
  // ...
}

and then linking the user command to the DLL function, as described in the Module SDK documentation: