how can i adjust heart rate used on apple watch workouts

I don't know of any way to adjust your heart rate for beta blockers. Ther only advice I can offer is to make sure your watch is calibrated and your personal information is up-to-date. See:

Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support

You can use the "other" workout type in the workout app on the phone,here's how it works;

Choose "Other workout type.With this workout you earn the credit equivalent of a "brisk walk" which means 1 minute credit for every 1 minute in the workout regardless of how fast or how slow you walk or run,this is particulary useful for anyone that can't walk at the brisk walking pace of 3.3 mph or are recovering from injury or are an older person who just can't reach the "brisk walking" speed.

So if you do 30 minutes in the "other" workout then you will close your green exercise ring.Active calories are also recorded and estimated at a rate equivalent to a "brisk walk" or based on data recorded by the heart rate sensor in the watch, whichever is higher.