We can only provision users on slack if your account is on the Plus plan or Slack Enterprise Grid.
We use the SCIM API to allow us to provision users.
To see what plan you are on with Slack click here: https://api.slack.com/admin/billing
If you are on a supported plan you should be able to create a Token on this page: https://api.slack.com/custom-integrations/legacy-tokens
Just paste that token and click connect.
If you get an error you can delete your tokens and regenerate. First go here: https://staffkeep.slack.com/apps/A02-slack-api-tester and delete the token. Then go back to https://api.slack.com/custom-integrations/legacy-tokens and create a new token to paste in.