I'm trying it Tomorrow. Do I warm up first or give meds first?

Following.  Let us know how it worked.  I've heard it is given prior to warm up.

All horses are a tad different you'll need to make minor adjustments to this. I use it about 1-1.5 hours out given IV. I just ordered the oral and will for the first time in my 35 or so years of training be trying it, I really hope I will like the oral administration as much as the IV. I have used it 2 hours out when I wanted a significant diuresis. If your aim is for a horse with pulmonary HTN I would start at the 2 hour mark. Your going to have a degree of sedation for approx. 3-4 hours. 

I warned her up then walked her to vet who administered .5 cc IV. I tied her to trailer and left her alone for 39 minutes then 15 minutes of walking before the run. She was like a different horse. She was calm and could think and listen. Now I have to learn to give IV shots. Hopefully, she won't need it for long.

Glad to hear you had some positive results. I'm excited to be trying the oral out. I will let you know how it works. Most of the time the horse realizes that they're going to be fine and you can wean them off.  

I give the oral version. 8mg/mL and I give 3.5 ccs to her 1.5 hours before her run. 

Have you ever used it IV or IM so you can compare the effectiveness and possible side effects?

I rode her with the IV guanabenz before I bought her. I told the sellers I wasn't comfortable giving IV injections so their vet found the oral compound for me. Sellers and vet figured out the oral dosing before she came to me. It has the same effect for this horse.