How do I create an auto fill field in Salesforce?

Auto–populate the lookup field with Process Builder

1. Step 1: Create a Process. From Setup, enter Builder in the Quick Find box, and select Process Builder.
2. Step 2: Choose Object and Specify When to start the Process. Click Add Object.
3. Step 3: Define Criteria. Click Add Criteria.
4. Step 4: Define Immediate Actions.
5. Step 5: Activate the Process