You've certainly got the right idea about what the IF statement does, ie, it is a logical test where you can specify a condition that you want to check (in this case, if the Activity Type is Proposal) and generate one value if the condition is met and another if the condition is not met (in this case show 1101 if the Activity Type is Proposal and show 1200 if the Activity Type is anything else).

So, you just need to amend the syntax of your formula slightly so that it looks like this:

IF(activity_type = 'Proposal', '1101', '1200')

All that's changed is that I've added the equals symbol.

Of course, you can use the same theory as shown above to generate all sorts of IF statements, by replacing Proposal with whatever data you want to check for and replacing 1101 and 1200 with the values you want to display if the condition is met or not.