How can I go about getting my CAB awarded retroactively?

You're gonna need some sworn statements at least.

THE CAB was to provide special recognition to U.S. soldiers who personally engaged, or are engaged by, the enemy. The CAB is intended to serve as a companion to the Combat Infantryman Badge (CIB) and Combat Medical Badge (CMB) and was created to recognize the greatly expanded role of NON-INFANTRY soldiers in active, ground combat. Actively engaging or being engaged by the enemy, and performing satisfactorily in accordance with prescribed rules of engagement after September 18, 2001.
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Eligibility requirements:
The Combat Action Badge may be awarded to any soldier not assigned or attached to a unit that would qualify the soldier for the Combat Infantryman Badge (CIB) or Combat Medical Badge (CMB) after the date of September 18, 2001, and:
The soldier must be personally present and under hostile fire while performing combat actions satisfactorily in accordance with the prescribed rules of engagement,
The soldier must be performing their assigned duties associated with the unit’s combat mission in an area where hostile fire pay or imminent danger pay is authorized,
For all named conflicts beginning after the effective date of this publication (5 March 2019), a soldier must also be performing in an offensive or defensive act while participating in combat operations, engaging, or being engaged by the enemy.
The CAB may be awarded to any army branch or military occupational specialty including infantrymen except when serving in a role where they would be eligible for the CIB. In addition to Army soldiers, the CAB may also be awarded to Servicemembers of other U.S. Armed Forces and foreign military personnel assigned to a U.S. Army unit, provided they meet the above criteria (for example, KATUSA in the 2d Infantry Division).
The Combat Action badge is unique in that unlike the Combat Infantryman and Combat Medical badges, it can be awarded to soldiers of any rank, including general officers, whereas the CIB and CMB are both restricted to colonels and below.
Award of the CAB is not automatic and will not be awarded solely based on award of the Purple Heart or being in the vicinity of combat by other friendly forces..

So it is not easy to do. But....
Get a copy of AR 600-8-22, it's available online. It's easier for you to look it up than cut and paste a bunch of pages.
It discusses retroactive awards. I'm working on one for a former Soldier.
-You will have to gather sworn statements from folks who were there. This is difficult once everyone is out of the service and getting a specific date/ time that the incident occurred can be hard (folks remember incidents but not the date). The sworn statements are the key.
- Contact your old unit and see if they have copies of the duty logs that list patrols , missions and enemy contacts ,that is helpful. (many if not all duty logs were classified so it takes time to have them redacted if they are available.)
- Did any of your buddies receive a CAB or Purple Heart for the same action you are discussing? If so, that helps to have a copy of their orders to substantiate the action.
Be prepared to answer the question; Why didn't the command put you in for the award?