You can apply an active template to a record using JavaScript.

Apply a template to current

To apply a template, use the applyTemplate method.
current.applyTemplate("<templatename>");

Apply a template to a GlideRecord

To apply the template to a record other than current, change current to a GlideRecord variable. When using a GlideRecord variable, you may need to initialize it after declaring the variable.

var rec1 = new GlideRecord("incident");
rec1.initialize();
rec1.applyTemplate("my_incident_template");

Apply a template from a UI action

The following script demonstrates a possible customization to the Create Change UI action on the Problem form. After you add this script to the UI action, a user can select the UI action to create a change record with information from both the problem record and the change template.
var change = new GlideRecord("change_request");
change.initialize();
change.short_description = current.short_description;
change.description = current.u_details;
change.cmdb_ci = current.u_service;
change.priority = current.priority;
change.requested_by = current.caller_id;
change.assignment_group.setDisplayValue('Change & Release');
change.u_status = 'New';
change.parent = current.number;
if(0 == change.applyTemplate("standard_rfc")) {
    current.rfc = change.insert();
    current.comments = 'Change ' + change.number + ' created.';
}

var mySysID = current.update();

gs.addInfoMessage("Change " + change.number + " created");
action.setRedirectURL(change);
action.setReturnURL(current);

Script a template with child templates

When using applyTemplate with a template that has one or more child templates, the system creates the parent record before applying the child templates. This behavior ensures that any references or dot-walked fields from the child record to the parent have a valid target.

For example, if a template for the Change Request table has a child template for the Change Task table, applying the Change Request template inserts a Change Request record into the database. It assigns this record as the Change request for the Change Task record, then applies the child template to the Change Task record.

Apply a template using a sys_id instead of a name

To apply a template using a sys_id, use the following method.
GlideTemplate.get(template.sys_id).apply(GlideRecord)