Open the layer to be extended in MapInfo Pro

 

Display relation list

 

Call the relationship structure in the context menu of the relevant level.

 

Add field

 

Select a suitable name for the column in the Name field and close with OK.

 

End action with "Close all

 

Reload the edited layer in CM

 

Now the additional field is available and can be filled with selected data.

This configuration is done in the library under MapInfo->CM Properties<->External Programs