You can’t set a specific password expiration date for a user in Active Directory. It’s not possible.
What you can do is set a Password Policy via Group Policy that applies to all accounts. I believe the default is something like 42 days. Previous NIST guidance was 90 days, but the current recommendation is "that businesses enforce password expiration and password resets only when a known compromise has occurred, or every 365 days " (source).
If you want to specifically set up a policy wherein new users need to change their initial password within X days of account creation, you can do this through a combination of Sailpoint-managed AD security groups and Fine-Grained Password Policies (which can be targeted at specific security groups).
Unfortunately, you can’t do date comparisons in a role assignment, but if you use Workflows to add newly-created AD accounts (trigger: Source Account Created) to a specific security group (action: Manage Access) that has a Fine-Grained Password Policy applied with a max password age of, say, 14 days, then have a separate workflow set up with a Source Account Updated trigger that removes the user from that security group when their pwdLastSet AD attribute changes, that should effectively let you ensure that users have a 14-day max password age just for when they’re newly onboarded, while still maintaining a general 90-day max password age for everyone else.
(note: when defining the fine-grained password policy, take into account how early before a user’s start date their account may be created; if accounts are created up to a week before they start, you will need to pad your max password age in your fine-grained password policy accordingly.)