The recommended way to set up an application username format in a scenario like this is to actually use the Profile Editor to set the username format.
To do this, follow the below steps:
Go to Okta Admin -> Directory -> Profile Editor
Locate the app that you would like to set the username format of
When you find that app in the Profile Editor list, click "Mappings"
Once the new window with your mappings appears, make sure to click the "Okta to APP" tab at the top of the window - the page defaults to "APP to Okta".
On the "Okta to APP" page you will likely see something that says "Username is set by APP - Override with mapping." You will want to click on this. Doing so brings up a box where we can then enter custom expressions for your username format.
Here you will want to enter in the correct attribute that you want to pass over for your username. For instance, if you want to use your Okta login, you could put "user.login" (without quotes). Email would be user.email, and so on. You can also use a custom expression if you prefer. There is a good list of expressions here: https://developer.okta.com/reference/okta_expression_language/index
After you set your username format, place a username in the "Preview" box in the bottom of the window to verify that the new mapping looks as you want.
Once this is confirmed, click "Save Mappings" and then immediately after, click "Apply Updates Now" if you would like to immediately re-evaluate usernames for your users. After doing this, the username should send as you want.
This is actually the preferred way to set up a username format for an app like this, and in the future our SAML apps will be more closely tied to Universal Directory for username settings.