Link Issue from One Issue type to Another Issue Type

[Spartans]

Hi Team,

I need a Help from your team for Automation for linking issue.

Story: 
I have like 4 Issue Type 
1. Source [Parent Issue]

2. CER [Child]

3. TER [Child]

4. PER [Child]

We are using Source as the Parent and others as child. Our Requirement was to link issue to the Parent Issue key which we mention. 

For Eg: If we have a Parent Issue key is PS-1 and if there is a issue created in other child, we will have separate custom field for Parent ID to be mentioned. 

If the issue created in Child and it matches the issue key, we need it to be linked to Parent issue as per the ID we mention. 

I tried Automation, but not finding the way exactly.

Is there any way for triggering it. Please help us identify the Automation or any other solutions