Custom actions are not available in the Essentials solution.
If you need to remove a user membership from all Microsoft 365 groups, you can import the following script in a CoreView custom action.
Custom action script:
{
"id": "1eba1dd4-e697-42b7-85d8-85acdd40e2c5",
"title": "Exchange - Remove a user from all M365 groups",
"description": "Remove a user's complete memberships from all M365 groups",
"lastModified": "2022-05-16T13:11:59.3820000Z",
"target": "None",
"tags": [],
"vars": [
{
"name": "UserPrincipalName",
"type": "string",
"isRequired": true
}
],
"params": [],
"columns": {},
"version": 4,
"statement": "param ([string]$UserPrincipalName)\r\n\r\n$user = Get-AzureADUser -ObjectId $UserPrincipalName\n\n$memberships = Get-AzureADUserMembership -ObjectId $user.ObjectId | Where-Object {$_.ObjectType -eq \"Group\"}\n\nForEach ($group in $memberships){\n Remove-AzureADGroupMember -ObjectId $group.ObjectId -MemberId $user.ObjectId\n}"
}