I saw from online videos (e.g. filling balloons with helium tank) that you have to hold the handle (not sure if that's called handle) with your hand to keep the helium gas flowing out. However, if I'm going to CTB, I wish to have the helium gas keep flowing out ("automatically") without holding the handle... Do you guys have any idea about how to accomplish that? Thanks.
The helium should be purchased from a welding supply company, because party balloon helium is almost always mixed with up to 20% air, making it not effective. Then purchase a combination flowmeter regulator that will attach to the type of welding cylinder you have, which will depend on what region of the world you purchase it. For instance, in the USA, inert gas cylinders have a CGA 580 type output port. If the flowmeter regulator doesn't have a barb-style output fitting for attaching PVC plastic tubing, an adapter can be purchased. Then the desired flow rate, such as 15 l/minute, can be set by turning the knob on the flowmeter regulator. Flowmeter regulators for helium, nitrogen, and argon can be purchased for less than $150 from welding supply companies.
Even if somehow a party balloon tank of helium could be found that was known to not be mixed with air, I would definitely not trust trying to hold the "bend" style valve open with tape, etc. due to the chance it would not hold, and not being able to set the desired flow rate.