Taurus pre-request authentication - performance

Question

I've faced a problem using Taurus. Could someone help me, please? I'm trying to simulate 300 users but before sending those 300 users POST requests, I need to generate a token. The token is attached to the request in that way:
- url: http://url?user_token=${auth_token}
Now I have the following scenario:
load_api:
requests:
- once:
- url: https://endpoint/authenticateUser
method: POST
headers:
Content-Type: application/json
body:
username: username
password: pass
generateToken: true
extract-jsonpath:
auth_token:
jsonpath: $.token
label: get_token
- url: http://url/user_token=${auth_token}
method: POST
headers:
Content-Type: application/json
body-file: test_data/body.json
label: sending_300
As you can see, a token is generated for each thread. And I need that it is generated before the script and then the token is attached to the URL as a parameter. I've tried to separate those for two scenarios but in that way variables from one script can't be used in the other. I was also looking at global variables but it seems like that kind of variable can be created only before executing. So, if someone could help me, I'll appreciate your time spent.
EDIT (thank you so much Dmitri T):
Here is a workable script:
execution:
concurrency: 300
scenario: load_test
scenarios:
load_test:
requests:
- if: ${__groovy(ctx.getThreadNum() == 0 && vars.getIteration() == 1,)}
then:
- url: https://url/authenticateUser
method: POST
headers:
Content-Type: application/json
body:
username: username
password: pass
generateToken: true
extract-jsonpath:
auth_token:
jsonpath: $.token
label: get_token
jsr223: props.put('auth_token', vars.get('auth_token'))
else:
- url: http://endpoint?user_token=${__P(auth_token,)}
method: POST
headers:
Content-Type: application/json
body-file: test_data/body.json
label: sending_300_reqs
think-time: 10s # waiter for processing auth request

Answer 1

If you want to generate a token once and share it across 300 threads:
Generate token for 1st thread during the first iteration using If block and convert it to JMeter Property in JSR223 block. The condition for the If block would be:
${__groovy(ctx.getThreadNum() == 0 && vars.getIteration() == 1,)}
and the code for JSR223 block:
props.put('auth_token', vars.get('auth_token'))
check out Top 8 JMeter Java Classes You Should Be Using with Groovy article to learn what these ctx, props and vars words mean
In your 2nd request refer the property using __P() function
http://url/user_token=${__P(auth_token,)}