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Info:

All log tables in Azure Monitor Logs must have a TimeGenerated column populated with the timestamp of the logged event.

Var1 - Azure CLI:

Choose the bash mode.

az monitor log-analytics workspace table create --resource-group <Resource Group Name> --workspace-name <Workspace Name> -n <Tabel Name> --retention-time <Numer of Days>  --columns <Column Name>=<Columntype> (e.g. Column1=string TimeGenerated=datetime)

Var2 - PowerShell:

Choose the PowerShell mode.

$tableParams = @'
{
    "properties": {
        "schema": {
            "name": "TEST2_CL",
            "columns": [
                {
                    "name": "TimeGenerated",
                    "type": "DateTime"
                }, 
                {
                    "name": "TestColumn2",
                    "type": "String"
                }
            ]
        }
    }
}
'@

Invoke-AzRestMethod -Path "/subscriptions/{subscription}/resourcegroups/{resourcegroup}/providers/microsoft.operationalinsights/workspaces/{workspace}/tables/MyTable_CL?api-version=2021-12-01-preview" -Method PUT -payload $tableParams

Var3 - Azure Portal:

Data Collection Endpoint:

Custom Log Table:

[
  {
    "TimeGenerated": "2014-11-08 15:55:55",
    "TestColumn3": "Irgendein Text"
  }
]

Additional helpful material:
Often you need a table, which matches the SYSLOG - table schematically.
Here you can find a script, with which you can create the table automatically.

$tableParams = @'
{
    "properties": {
        "schema": {
            "name": "TEST2_CL",
            "columns": [
                {
                    "name": "TimeGenerated",
                    "type": "DateTime"
                }, 
                {
                    "name": "TestColumn2",
                    "type": "String"
                }
            ]
        }
    }
}
'@

Invoke-AzRestMethod -Path "/subscriptions/{subscription}/resourcegroups/{resourcegroup}/providers/microsoft.operationalinsights/workspaces/{workspace}/tables/MyTable_CL?api-version=2021-12-01-preview" -Method PUT -payload $tableParams

References:

Add or delete tables and columns in Azure Monitor Logs - Azure Monitor
Create a table with a custom schema to collect logs from any data source.
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