obsidian.templater.date-basis.js

<%*
const dateFormat = "YYYY-MM-DD";
const dateBasisStr = await tp.system.prompt("When is the meeting?");

if (dateBasisStr) {
    function createMessage(daysBefore, dateFormat, dateBasisStr) {
        let parseResult;
        let newDate;
        let parseResultLink;
        let nlDatesPlugin = app.plugins.getPlugin('nldates-obsidian');
        parseResult = nlDatesPlugin.parseDate(dateBasisStr);
        newDate = window.moment(parseResult.formattedString).add((daysBefore * -1), 'days').format(dateFormat);
        if (newDate) {
            if (newDate !== 'Invalid date') { parseResultLink = '[' + '[' + newDate + ']]'; }
        }
        if (parseResultLink) {
            return parseResultLink;
        } else {
            return "";
        }
    }

let msgDate;
msgDate = createMessage(14, dateFormat, dateBasisStr);
tR += `Send agenda on ${msgDate}`

//Repeat to add multiple lines / dates
msgDate = createMessage(7, dateFormat, dateBasisStr);
tR += `\n`;
tR += `Send reminder on ${msgDate}`

msgDate = createMessage(1, dateFormat, dateBasisStr);
tR += `\n`;
tR += `Tomorrow is the meeting! ${msgDate} will remind me!`

}
%>

The next challenge would be if you wanted to generate more than one line with different dates relative to the date basis.

@kmaustral I just updated the code to allow for adding multiple lines with different dates and messages. It uses a function now so can just repeat as few or many times as you want by copying these 3 lines and repeating:

//Repeat to add multiple lines / dates
msgDate = createMessage(7, dateFormat, dateBasisStr);
tR += `\n`;
tR += `Send reminder on ${msgDate}`