If I want to make hot chocolate or coffee, can I put the spoonfuls of chocolate/coffee directly into the flask with hot water, close the lid and shake it about?

Or do you have do prepare the drinks in a separate container before and then pour it in?

My thermos flask is metal, so doesn't have glass in it.

I always make it in a large jug and pour it in. That way I can see if it's right and not too strong or weak.

Always fill your Thermos with really hot water a little while before you are going to fill it with your drink of choice.
It'll stay hotter for longer 👍

I get that. But is there anything wrong with making the drink directly in the flask rather than making it elsewhere and then pouring it in?

Shouldn't think so, as long as your able to make the drink to the desired strength you like, inside the flask.
No good putting teabags in and leaving them floating inside it for hours though

Nothing 'wrong' with it, but I imagine you'll be able to prepare the drink better in a jug than a flask. Hot chocolate, for e.g., is all about the mixing (or whisking) to ensure the powder disolves fully in the water. Easy in a wide, transparent jug, much less so in a tall, narrow, dark flask, and no-one wants powdery hot chocolate !

One weird phenomenon I've discovered is that if you prefer sweeteners to sugar, they somehow seem to lose their sweetening effect after an hour or two in a flask. It's best to take some with you and put them in the cup just before you drink it.

Yes but it will have cooled down slightly , before you finally pour it into your flask
I now just fill the flask with boiling watet, and carry instant mocha or latte and make from
Fresh... even carry t bags just add the hotwatet and seperate milk

Finally?

Boiling water doesn't cool that much in a half a minute or so.

It's still too hot to drink straight away when you pour it out.

Heat thermos and jug with boiling water.

Make drink in jug and taste (esp if the drink is sugared as you can misjudge it when making an unusually large volume).

Discard the hot water from thermos. Pour drink in thermos asap and get the stopper in asap.

It must be 30 years ago but I used to just put tea leaves, boiling water and then milk into a Stanley insulated flask (metal lining).

Seems odd I know, you'd think it would taste stewed, but I found it fine.

With hot chocolate or coffee I'm sure you'd get to know how much to use pretty quick.

I used to warm the flask with boiling water first.

Never, ever put the milk in at the same time as the tea. Always carry it separately and add it afterwards. Some flasks even have a compartment at the bottom with a specially designed milk bottle so you can do this.

Yes it does alter the taste if you put milk in with the tea.
My own theory is that the milk cooks and breaks down, rendering the drink tasting sweeter.
I actually don't mind that - I just think of thermos tea as tasting different to ordinary tea.

If I wanted ordinary tea, I would do as you suggest and convey the milk separately.

Me too. But I heat it up with hot water first.

My flask holds 3 mugs full of coffee. So I put 3 spoons of coffee granules in the flask, pour in 3 mugfuls of boiling water, add milk to reach the right colour / strength, and stir it up with a long handled ' sundae ' spoon. Take my Hermesetas and a spoon with me as only me likes it sweetened.

I make a jug of tea for my flask

A couple of tea bags, add boiling water, stir about a bit, remove bags, add milk, pour into flask which has been warmed with water from the kettle.

I always over estimate, so I drink the remains whilst faffing about before leaving the house.

I use a metal flask as I have broken a couple of the other type in the past, and the mess, combined with a lack of tea is distressing

Stays nice and hot for a midday slurp.

Prepare Thermos with boiling water.

I put milk in a jug and microwave to very hot.
Then I add the instant coffee to the milk - too hard to get the granules into the (now wet) flask
Pour milk into emptied thermos.
Top up with boiling water.

Stays hot all day.
My all metal flask cost me £3 from Asda (others are available) that is the price of one cup of coffee at a motorway services

I just fill my flasks with water and pour it into my normal mug to see how many mugs I get so I know how much coffee to add, then add boiling water give a quick shake and I'm good to go.