Insert → Table → 3x1.
Right click table → Table Properties.
In Table Tab, Check Preferred Width → Percent → 100.
In Column Tab, set preferred width to 7%, 86% and 7% for 1st 2nd and 3rd column respectively.
These values work well for Times New Roman 12pt equation numbers. (Other percentages will work provided they add up to 100%.)
Click Ok.
Insert → Equation into center column (type in current equation or placeholder).
Click References → Insert Caption.
Select Label: Equation
Position: Above or Below
Adjust numbering as desired.
Cut and past number from above location to right column of equation table.
Right-align text in right column.
Center equation column.
Highlight the entire table.
Turn off borders.
Re-highlight the entire table.
Select Insert → Equation → Save Selection to Equation Gallery.