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.