Use the zig zag stitch , typically I use the widest width. Take the stitch lenght down to zero or .5. Take several stitches inplace. You can leave long top and bottom threads and using a needle bring the top thread to the bottom and make a square knot with the bobbin thread. FYI ...I almost never do this , taking several stitches is sufficent to hold the tack.
I have also done a tack using a decorative stitch on my machine , stitching only one motif , but that does require knotting the two threads together.
Some use a dot of fray check on the tack, I can go either way on the fray check... as it can migrate a bit and it darkens the place that its used. If you do fray check the tack use a very small opening on the bottle , and just dot both sides of the tack.