First try this.
Go to:
https://developer.paypal.com/
Login with your usual paypal details
Click on the button icon in the top-right corner of the page and select 'Dashboard'
You should see a blue button - 'Create App' - click it
In the 'App Name' field which will appear on the page AFTER you have clicked the 'Create App' button just give the app a name, anything you like - example - myTestApp
Then click on the blue 'Create App' button at the foot of the page.
On the page that appears you should see:
Client ID - a long string of characters - that is the id I believe you need to use instead of 'sb' in the paypal link.
You may need to set up a Sandbox testing account if you have NOT already done so OR a couple of default accounts may already be set up for you to test with.
Click on 'Accounts' in the blue left sidebar, then when the next page appears click on the blue 'Create Account' button.
From the modal which drops down select 'Business (Merchant Account) - select your Country/Region from the drop select box, click the blue 'Create' button. Your new test business acoount will be created.
As I say you may not even need to go through the setting up an account stage to get a client id.
Further to the above and on reflection just create a 'live' app.
Select 'Dashboard' from the list under the button in the top-right corner of the page on the pay pal 'developers' page.
That should bring up a page:
My Apps & Credentials
There will be 2 buttons - 'Sandbox/Live'
Select 'Live'.
Click on 'Create App'
Give the app a name when the next page appears.
Click on the blue 'Create App' button
A page should appear:
LIVE API CREDENTIALS
Your live 'client id' will be listed - a long string of characters - that's the one you need for your live production, paste it where 'sb' is in the link to pay pal.
As an aside note:
Paypal never used to be this complex. In the past you could just create simple form based buttons in the paypal user area for static websites. Does that option not exist any longer for you? I'm using an old account so for old accounts that option may still exist but not for new ones.
On the other hand you may be trying to do something that can't be done with a simple button but can be done with the more complex javascript workflow - I dont know.
See the below youtube video for a visual represenation of what I've gone through in the above 2 posts. Watch it all the way through - the tutor eventually creates a 'live app' as I explained above and has access to the live 'client id'
https://www.youtube.com/watch?v=_haQPsNjq8M&t
In the past Ive never bothered with Sandbox as it was a nightmare at times (may have improved in recent years). I just went live and paid a minimal transaction amount like 1 pence to test.