Rather than write a long answer, I have decided to move this to a blog post. Below is the result we are after. We want to add multiple checkbox controls inside a DataCard. Each selected checkbox will populate a Text Input control, and each Checkbox which is unchecked will also remove that selection from the Text Input Control.
The solution has been tested with a standalone Canvas App, and verified with either SharePoint or Dataverse as a data source.
Solution One: add individual checkboxes into a DataCard
1. Add an EditForm Control into your App
2. Add your Single Line Text field into the Form if not already present
3. Unlock the DataCard for that field
4. Insert five Checkbox controls into the DataCard. Arrange them as desired and expand the height of the Datacard to accommodate the Checkboxes
5. For this example, we are going to name each checkbox control "CheckboxMonday", "CheckboxTuesday", "CheckboxWednesday" etc
6. Update the Text property of the Checkbox by removing the default text (“Option”) and replace with “Monday”, “Tuesday” etc
7. On the Default property for each Checkbox control, enter:
Self.Text in ThisItem.'Your single line text field'
8. On the Default property of the DataCardValue (the Text Input Control), enter: