How Do I Set A Default Value for a Radio Group?

I’ve got a Radio Group that I want to start up with a default value… so I want the first option blank_message to be set.

I’ve put checked="checked" in this input element but that doesn’t do it… what is the best way to do this?

<div is="dmx-radio-group" id="i_message_start_with" class="radio-group">

									<div class="form-check form-check-inline w-100">
										<div class="row c_right6 w-100">
											<div class="col px-1">
												<input class="form-check-input" type="radio" value="blank_message" id="radio_blank_message" name="start_with" checked="checked">
												<label class="form-check-label" for="radio_blank_message">Blank<span dmx-hide="browser.viewport.width&lt;576">&nbsp;message</span></label>
											</div>
											<div class="col px-1">
												<input class="form-check-input" type="radio" value="existing_message" id="radio_existing_message" name="start_with">
												<label class="form-check-label" for="radio_existing_message">Existing<span dmx-hide="browser.viewport.width&lt;576">&nbsp;message</span></label>
											</div>
										</div>
									</div>
								</div>