Casting on in middle of purl row

I need to cast on stitches in the middle of a purl row. Please provide instructions; I’m sure it will be different than casting on in middle of a knit row.

Thank you.

Welcome to KH!
You can use the same cast on and in the same way. A cable or knit cast on will work.

How many sts do you have to cast on?

I have 45 stitches to cast on. Again, this is written as doing so on the purl row.

Yes, so a knit or maybe even better, the cable cast on. If it were only a few sts you could get away with a backwards loop but that’s not such a good cast on for 45sts.