she gave me her number, over the internet, obviously shown an interest in talking to me. 3 weeks later, I texted her by saying "SURPRISE!!!" as an opener.

I like her but am confused if she likes me since she gave me her number and showed some indicators of interest, like, saying "haha" a lot (sometimes in capital letters), are exclamation mark counted as IOI?, often replies in full and long sentences and very aware of our conversations.

however, apart from those signs, she doesn't seem to invest her time into texting me and often replies to my texts 6 hours or a day later. this makes me feel like my texts didn't make much impact on her.

I made a joke the other day:
-me: were you being kidnapped or abducted by aliens yesterday? (alien emoticon)
-C (she's 18): haha, I wish it was so sensational. was just busy! you? having exams, right?
-me: blablabla
-C: (didn't reply)

I didn't ask her what she's busy with 'cause I would come up as clingy if I said that.

so is there a way that I can make her invest her time into talking to me more

Why try to chat with a person who doesn´t have the time for it?

Why would she be invested in you if you waited 3 weeks to text her?

Answer these questions and you will probably see for yourself where you´ve gone wrong.

Set up a date, face to face with her, right now, by text or calling. If she declines, move on. You´re not supposed to bang every girl you talk to and it is unhealthy to cling on to every girl you see in favor of that goal.

_________________
No matter what, I must live without regret

Life is good

Live it for yourself