I assume that you do not want to simply set an association between your tender and your template text, for which you could simply set a drop-down Reference selector...
Instead, you need to pass both your tender object and the template object selected to a microflow. I assume you already have a data grid form showing the templates to select from. Add a data view of the tender object to this form (you do not need to add any fields from the entity to display - just the empty data view). Then move your templates data grid inside the tender dataview. The microflow on the button that opens the template list will now need to pass the current tender record as a parameter.
Then add a microflow action button to your templates data grid which should now have 2 parameters - one for the enclosing tender, and the other the selected template. In the microflow you should set the attributes in the tender from the selected template, refresh the tender, and add a close action to close the popup.