How can I ensure my doctor will allow delayed cord clamping?

Why. You can request a delay in cord clamping but be aware that there is a downside to prolonged delays. Although baby can benefit from the blood returning from the placenta, it can also come across & thicken the blood enough to cause kidney damage or a stroke. A blood test is routine to assess the thickness of babies blood & done after 2-4 hours to look for this.
Mention it now. You should discuss this with your doctor now and then before the baby is born. I will say that there are some rare situations where it might be better for the baby not to do this. For instance if there is any concern about the baby's heart tracing right before delivery, if there is any meconium at delivery, or if the baby is not transitioning well then they may need to cut the cord sooner.