Explanation/Reference:
Create the NAT policy.
1. Select Policies > NAT and click Add.
2. Enter a descriptive Name for the policy.
3. On the Original Packet tab, select the zone you created for your internal network, here Trust-L3, in the Source Zone section (click Add and then select the zone) and the zone you created for the external network, here Untrust-L3, from the Destination Zone drop down.
4. On the Translated Packet tab, select Dynamic IP And Port from the Translation Type drop-down in the Source Address Translation section of the screen and then click Add. Select the address object you just created.
5. Click OK to save the NAT policy.
References:
https://www.paloaltonetworks.com/documentation/60/pan-os/pan-os/getting-started/configure-nat-policies