MariaDB (mysql like) How to get the index value of a selected row in a sql table? This the unique index, not a row count

Say you have a table… Index is primary key…

Index Sensor
1     BME280
2     BMP280

And want to return the Index value only of a matching select query?

SELECT * FROM Sensor WHERE Sensor = ‘BME280’ Will return…

Index Sensor
1     BME280

But if I only want the index?

I could have sworn in the past I could use ROWid or INDEX() or such? But now having not done SQL work for a while, I can seem to get only the index returned for the matching row? What am I forgetting?