How can I get option 40 in my contract being prior service?

At MEPS the career counselor will go over available options. But he may not be forthcoming about "all" available options. Option 40 may be something they can get you second or third quarter this year, but they do anything they can to sell recruits on something up front that does not require delayed entry.
If Option 40 is not available at all this FY, then get 11B with airborne school guaranteed and volunteer for RASP at airborne school. If they aren't taking volunteers while you are there, then keep volunteering for RASP from your first unit. If you don't get it by the time you make E5, go to Ranger school, then keep volunteering for RASP.

Just ask. If they say no, then just ask for 11B airborne and volunteer for RASP in Airborne school.

I always had trouble with prior service when I was a recruiter, you are needs of the Army. The way the Army sees you, is as a Soldier who holds an MOS and is ready to PCS ASAP. If you really want the tab, get back in, fill up that 4187 and request a school date. Once you graduate, branch can find you a slot in a regiment. I always tried to explain to prior service, you cannot demand anything. I had more than one tell me "if I don't get what I want I won't sign" I always told them, you know where the door is. Good luck in your future endeavors, hope you get it.

I basically did the same thing. I got no options as prior service coming over to active. I volunteered for Airborne, then volunteered for the Rangers at Jump school. I am not saying this is the best option, but there are other options if you cannot get it in your contract.

As a prior service you are restricted by USAREC Business Rules. It tells what is available for prior service. Just be advised that preference is to send you with your already qualified MOS. If not then the business rules she’s what MOS are available. Nevertheless, the REI answer for your question with the Guidance Counselor at MEPS. S/he has the latest and greatest potentially could make a phone call to the right folks!

Option 40 is probably not in the cards for you. Your best bet if you want Ranger is to enlist for 11b as others here have said. Make sure that you get the guaranteed Airborne into your contract. While there volunteer for Ranger school. The Ranger recruiters come around to the Airborne school and look for recruits.

Alternatively you could talk to a counselor at MEPS. Until you sign that dotted line you are not locked into a blasted thing.

So- here’s the thing. You are prior service which means you are at the needs of the Army and therefore you pretty much have the option of taking or leaving what opportunity they offer you. Chances are option 40 isn’t going to be one of those options. You can certainly request when you get to where you are going to go to ranger school.
Hate to be a buzz kill but that is reality.

Why option 40? Just try to go 18X. It’s more likely to get 18X than an option 40

Your recruiters may be able to request it be added to your contract