I believe there is an O365 Office workflow action that will allow you to adjust item permissions on-the-fly for a list item - but it will require a password to be hard-coded into the workflow - meaning it will have to be updated regularly if you have any kind of password refresh policy.

Hello.

Yes such an action exists, it's name is Office 365 update item permissions, but this action is only able to change the permissions on item-level, not on List level.

So I think the only way to change the permissions on List level is by using the SharePoint web services by using the action web request

Regards Oliver

Thanks Oliver. Any idea how to do this with a web request action? (I've never used this action before)

Thanks

Hello, yes you can google SharePoint wdsl list permissions.

Methods don't be afraid, the methods still valid.

Link to the WSDL definition https://YOURTENANT.sharepoint.com/sites/[yourSITECOLL]/_vti_bin/Permissions.asmx

Hope it helps

regards

oliver