Hello Gert,
I recommend you review the Data Grid - Templates demo. Pay your attention to the Availability column and how the EditTemplate template for this column is implemented. You can use the same approach with the default HTML element:
<DxDataGridColumn Field="@nameof(WeatherForecast.Summary)" Caption="Summary">
<EditTemplate>
@{
var value = ((CellEditContext)context).CellValue.ToString();
<textarea @onchange="@((arg) => ((CellEditContext)context).OnChanged(arg.Value))">@value</textarea>
}
</EditTemplate>
</DxDataGridColumn>
Let me know if this information helps.
Regards,
Vova