You need to speak to your prescribing doctor before you start taking it. It is quite likely they will say don't start taking it while pregnant.
The problem is that the safety of ADHD medication during pregnancy isn't well documented. As such they tend to treat it on a basis of harm reduction. If the harm to you/your baby would be greater by being unmedicated because your unmedicated behaviour is very harmful (e.g. risky behaviour, drug abuse, severe issues with diet etc) then they would look to continue medication perhaps with a lowered dose during pregnancy, and taper down towards the end because you don't want the baby to be born with an addiction.
But it doesn't make sense to start a new medication when pregnant because the risk of the medication is greater in that case.
I would be quite tempted to just pretend I was taking it and stockpile during pregnancy
This seems like a spectacularly bad idea - don't do this. Aside from anything else OP has not yet begun titration so it would be complete guesswork and how would you explain this later on if you didn't turn out to get on with the first medication given?
Sorry I know it sucks if you have been waiting a long time for ADHD medication. I was actually in the same situation - fertility issues meaning I had been TTC for several years, I got pregnant the week after getting my ADHD diagnosis, although then had a miscarriage (touch wood this won't happen to you!) but I informed them (perhaps unwisely!) that I would be continuing to TTC. They then said in that case, we won't pursue medication at this time but after you've had the baby, this is what to do.
It was actually about 7 years in total between my diagnosis and getting medication partially for this reason, partially because it took a while to conceive again, partially because I didn't get around to sorting the new appointment. It was still helpful to have had the diagnosis within that time, it meant I could research and join support groups and so on.