Auto Populate one field Based on Values in Another Custom Field

[Deepa Tantry]

Hello all,

I have a single select list field and a group picker field. Values of the select list field is exactly same as the group names that is selected via the group picker.

Requirement : When selecting a value in the select list field, for example "Jira-dev", the group picker field should auto populate the group "Jira-dev".

The select list field exactly contains the group names as a list of values(text). 

Need of the requirement: We have 15000 groups in our instance. We have a need in one particular project, that the group picker field should only list the groups(around 2k in count) related to that project. 
We have an API developed that contains the list of groups(2k) as text value names, required to be listed in the group picker field.
The select list field is an nfeed field, and the list of values is populated from an API. 
Currently when using the field we get the group names in the list to select, however its just a text value. 
So, we would have to auto populate the group picker field based on this select list field. This is required because the permissions to view the ticket is restricted to the group selected.

Hope i am clear, if there is any clarifications required please update.

Can this be done via Groovy/behavior script that can be triggered during a transition.
Please let me know all possibilities that we can try implementing.

Thanks,
Deepa

[Alexey Dorofeyev]

I think you need something like this

https://community.atlassian.com/t5/Jira-questions/Conditionally-set-field-value-based-on-other-field-value-on/qaq-p/460864