//To specify a custom ID in the i18n attribute or $localize tagged message string, use the @@ prefix. The following example defines the introductionHeader custom ID in a heading element.
<h1 i18n="@@introductionHeader">Hello i18n!</h1>

variableText1 = $localize `:@@introductionHeader:Hello i18n!`;

//The following example includes a description, followed by the custom ID.
<h1 i18n="An introduction header for this sample@@introductionHeader">Hello i18n!</h1>

variableText2 = $localize `:An introduction header for this sample@@introductionHeader:Hello i18n!`;