Currently there's no an action to do this, but with some work you could use the action to call a web service and user the REST SharePoint methods.

Something similar to this >>> Set custom permissions on a list by using the REST interface | Microsoft Docs 

Thanks for your response.

So I did three HTTP requests :
1]  Get my group ID :
GET
<site_url> /_api/web/sitegroups?$select=Id,Title&$filter=Title eq '<GroupName>'

2] Break role inheritance on my list :
POST
<site_url> /_api/web/Lists/Getbytitle('<ListName>')/breakroleinheritance(true)

3] Add my group permission to this list :
POST
<site_url> /_api/web/Lists/Getbytitle('<ListName>')/roleassignments/addroleassignment(principalid=<GroupID>,roledefid=<RoleDefinitionID>)


Note : Thanks to 'SviPullo' SviPullo: SharePoint RoleDefinition Ids

Role Definition Name      |      Role Definition Id
Full Control                            1073741829
Design                                    1073741828

Edit                                         1073741830

Contribute                              1073741827

Read                                       1073741826
View Only                               1073741924

Hi ‌, is your approach working fine as what you expected?

Yes this approach work fine .

Can you send a user voice to your engineer service, to add this new action "Set List permission" ?

It's important for us and our clients to manage list permission fastly, so if the Nintex product can do this, it will be great !

Hi, I was curious if this method could be used to set permissions on a single list item. We cannot use the O365 update item permissions action due to the password being unencrypted. 

Thanks

Yes you can use this method but see the difference in the second request below :

1 - <site_url> /_api/web/Lists/Getbytitle('<ListName>')/items('<id_item>')/breakroleinheritance(copyRoleAssignments=false, clearSubscopes=true)

2 - <site_url> /_api/web/Lists/Getbytitle('<ListName>')/items('<id_item>')/roleassignments/addroleassignment(principalid=<GroupID>,roledefid=<RoleDefinitionID>)

Thanks Ivan for taking the time to help me out.

I see in #1 that the inheritance is being cleared out on an item in a list.

In #2 it looks like the assignment is being added back in but for a group not for an individual user name.

For example if I wanted to break the perms and then set the perms on a list named testlist to read only on a single list item for myself it would look like this?

1 - ')/roleassignments/addroleassignment(principalid=stirns,roledefid=1073741826)

Thanks

Steve

To add the current user or another user permission to the list item , if i see the documentation of Microsoft, you can put the UserID or GroupID.

See the documentation here.

So for example : 
/roleassignments/addroleassignment(principalid=XXX,roledefid=1073741826)

Get the UserID by LoginName :

If you don't have the information of your UserID but only LoginName, you need to do a call HTTP GET, to get the UserID by your LoginName.

HTTP GET
<site_url>/_api/web/siteusers(@v)?@v=%27i%3A0%23.f%7Cmembership%7Cjohn.doe%40contoso.onmicrosoft.com%27

WARNING : You need to have the LoginName Encoded ! Very Important ! If not, don't work !
Example : 
i:0#.f|membership|john.doe@contoso.onmicrosoft.com
to

%27i%3A0%23.f%7Cmembership%7Cjohn.doe%40contoso.onmicrosoft.com%27

Voilà !  

Thank you very much I will test it out soon