How can I phrase this to make it sound professional

Start your sentences with "As a professional..."

As a professional in the field myself, your company's incentive plan caught my attention as I was browsing the web. I feel that it would be challenging and motivate me to work harder

I had an answer all ready but then I thought "Why would I help someone who is thinking of voting for that creature Trump?".

If the voting for Trump thing is an A2K April Fool joke, it is going down like a lead balloon, and is as stupid as the Google one.

Thank you so very much for your help! I get the idea now. Really appreciate it

Actually the field of work I'm applying for is a different one than the one I'm in. In that case, should I just take out the "in the field myself" part?

I'm currently working a part-time job and this one I'm applying for will also be part-time.

Thank you for the tip, Robert. I appreciate it.

Maybe "...in the field of____..."

Bob, help

Would a company be more likely to hire someone whose primary motivation is incentives or someone whose primary motivations are an interest in the work and a desire to do something worthwhile?

It's not an either/or issue. When I am considering a candidate I know they want the incentives; the reason we offer them is to get the right candidates to apply and subsequently stay. I don't, within limits, give a damn about what is their "primary motivation". I can detect bullshit a mile off.

Well now I feel bad for being facetious, but if you want honest advice don't worry about sounding professional so much as having the right message to deliver in the first place.

And someone else touched on what I think is the right approach here. Put yourself in their shoes and think about what they are interested in in your business relationship. Then shape the message that way, to be about what they want and not about what you want.

Productivity is the name of the game, not verbal bragging of what you might do.
An applicant who has looked into the company, and understands their product and/or services it provides is one step ahead.