Are you interested in creating a custom field for deals that could auto-generate a unique ID? Sometimes they are called project numbers, project IDs, job numbers, job IDs, reference numbers, reference IDs etc. Some companies use an alphanumeric number and some just use a numeric value.
To the best of my knowlege Hubspot doesn't currently have a custom field that can be configured to auto-generate a value. Previously they have recommended using the deal ID in the URL string but this 8-digit number is not searcheable and also hard to remember as other users have pointed out.
If you're interested in an idea like this vote it up in this thread and/or the following threads: