Thank you all for the advice. I like the gift bag idea with the donation in a book inside.
I am meeting a provider for drinks, dinner then smiles after. When should I take care of the donation? When first meeting in the bar or once we get to the room?
The first thing that I would do once we got to the room would be to place the envelop in plain view. On that note, one of the biggest turn offs for me is when a provider checks to see what is in the envelop. Usually they don't at all, but sometimes they do when I am in the bathroom -- for one reason or another.
Why do you care if they do it while you're in the bathroom?
You should not be giving advice yet.
Most providers expect their donation at the beginning of the date. In this case the OP was asking about meeting for dinner prior to bedroom fun. For most providers receiving the donation when getting to the room is a bit late.
If that is your fantasy, and her making sure she gets paid brings an unwanted dose of reality to the whole thing, I suppose I understand. BUT most ladies want to get paid at the start of the date and if your date starts in public, that's when she will expect to get paid, unless of course you and her already have a comfort level that precludes her worrying about getting paid.
Fish gave excellent advice about reading her website, many ladies who do a lot of public/private time dates will specify how they prefer for the donation to be handled. This is also one of those few times where an envelope should be used for discretion as simply handing her a wad of cash at the bar or dinner table would be the ultimate in indiscretion. lol
Lastly PS is absolutely correct that with barely a month of experience under your belt, it might be wise to spend a few more months asking questions before attempting to answer them. If you add up the combined experience of the other four guys (including me) responding to this thread; PS, BPS, Fishbro and myself, we have close to 150 years combined experience doing this. You've been here about 45 minutes. It's kind of like trying to fix your own plumbing with no experience, all you end up doing is making it more difficult for the professional who has to come in and fix your mess.
...I once met a provider in a bar and I noticed she was wearing running shoes. She looked a lot faster than me so I didn't give her the donation until we got to the room.*
* JK! 
you can give her a book with an envelope inside... I love to read!
Often gals have specific instructions on how and where to leave the envelope. Follow them precisely.
If none, then as suggested, place in an obvious spot up in her room. I often place it down on her laptop computer if she has one.
In the absence of that, you can always bring a little gift bag, or a greeting card, with the donation inside. Since it is totally normal for people to have dinner because they are celebrating something, one party handing a gift bag, or card to the other party never looks inappropriate. (But as GaG said, handing a wad of cash over would definitely raise eyebrows.) A book as TV mentioned, is also an excellent choice.
If it's a lady you already know and have spent time with in the past, it might be ok to handle the fee once you get to the room, but for a first meeting, she will definitely expect to get paid upfront, and discreetly.