Click "Settings" in the Georeferencer and go to "Transformation Settings…"

In the drop down for "Transformation Type" select "Polynomial 1"

In the drop down for "Target CRS" select "Project CRS: EPSG:3857 - WGS 84 / Pseudo-Mercator"

Click "Project CRS: EPSG:3857 - WGS 84 / Pseudo-Mercator"

Click the boxes on for:\
"Use 0 for transparency when needed"\
"Save GCP points"\
"Load in project when done"

\
Click "OK"

Click "Use 0 for transparency when needed"

QGIS Georeferencing Tutorial 8: Adjust Georeference Settings